Reapply ccddb6ffad1, "Emit a worst-case prologue_end" #116084
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In 39b2979 Pavel has kindly refined the implementation of a test in such a way that it doesn't trip up over this patch -- the test wishes to stimulate LLDBs presentation of line0 locations, rather than wanting to always step on line-zero on entry to artificial_location.c. As that's what was tripping up this change, reapply. Original commit message follows. [DWARF] Emit a worst-case prologue_end flag for pathological inputs (llvm#107849) prologue_end usually indicates where the end of the function-initialization lies, and is where debuggers usually choose to put the initial breakpoint for a function. Our current algorithm piggy-backs it on the first available source-location: which doesn't necessarily have anything to do with the start of the function. To avoid this in heavily-optimised code that lacks many useful source locations, pick a worst-case "if all else fails" prologue_end location, of the first instruction that appears to do meaningful computation. It'll be given the function-scope line number, which should run-on from the start of the function anyway. This means if your code is completely inverted by the optimiser, you can at least put a breakpoint at the _start_ like you expect, even if it's difficult to then step through. This patch also attempts to preserve some good behaviour we have without optimisations -- at O0, if the prologue immediately falls into a loop body without any computation happening, then prologue_end lands at the start of that loop. This is desirable; but does mean we need to do more work to detect and support those situations.
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@llvm/pr-subscribers-debuginfo @llvm/pr-subscribers-backend-x86 Author: Jeremy Morse (jmorse) Changes[This is a PR to trigger the precommit CI, this commit is a reapplication of #107849, hence the skip-approval label] In 39b2979 Pavel has kindly refined the implementation of a test in such a way that it doesn't trip up over this patch -- the test wishes to stimulate LLDBs presentation of line0 locations, rather than wanting to always step on line-zero on entry to artificial_location.c. As that's what was tripping up this change, reapply. Original commit message follows. [DWARF] Emit a worst-case prologue_end flag for pathological inputs (#107849) prologue_end usually indicates where the end of the function-initialization lies, and is where debuggers usually choose to put the initial breakpoint for a function. Our current algorithm piggy-backs it on the first available source-location: which doesn't necessarily have anything to do with the start of the function. To avoid this in heavily-optimised code that lacks many useful source locations, pick a worst-case "if all else fails" prologue_end location, of the first instruction that appears to do meaningful computation. It'll be given the function-scope line number, which should run-on from the start of the function anyway. This means if your code is completely inverted by the optimiser, you can at least put a breakpoint at the start like you expect, even if it's difficult to then step through. This patch also attempts to preserve some good behaviour we have without optimisations -- at O0, if the prologue immediately falls into a loop body without any computation happening, then prologue_end lands at the start of that loop. This is desirable; but does mean we need to do more work to detect and support those situations. Patch is 23.44 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/116084.diff 9 Files Affected:
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 9f872b75207c05..65be55b2840c32 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2062,6 +2062,16 @@ void DwarfDebug::beginInstruction(const MachineInstr *MI) {
unsigned LastAsmLine =
Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine();
+ if (!DL && MI == PrologEndLoc) {
+ // In rare situations, we might want to place the end of the prologue
+ // somewhere that doesn't have a source location already. It should be in
+ // the entry block.
+ assert(MI->getParent() == &*MI->getMF()->begin());
+ recordSourceLine(SP->getScopeLine(), 0, SP,
+ DWARF2_FLAG_PROLOGUE_END | DWARF2_FLAG_IS_STMT);
+ return;
+ }
+
bool PrevInstInSameSection =
(!PrevInstBB ||
PrevInstBB->getSectionID() == MI->getParent()->getSectionID());
@@ -2138,32 +2148,109 @@ static std::pair<const MachineInstr *, bool>
findPrologueEndLoc(const MachineFunction *MF) {
// First known non-DBG_VALUE and non-frame setup location marks
// the beginning of the function body.
- const MachineInstr *LineZeroLoc = nullptr;
+ const auto &TII = *MF->getSubtarget().getInstrInfo();
+ const MachineInstr *NonTrivialInst = nullptr;
const Function &F = MF->getFunction();
// Some instructions may be inserted into prologue after this function. Must
// keep prologue for these cases.
bool IsEmptyPrologue =
!(F.hasPrologueData() || F.getMetadata(LLVMContext::MD_func_sanitize));
- for (const auto &MBB : *MF) {
- for (const auto &MI : MBB) {
- if (!MI.isMetaInstruction()) {
- if (!MI.getFlag(MachineInstr::FrameSetup) && MI.getDebugLoc()) {
- // Scan forward to try to find a non-zero line number. The
- // prologue_end marks the first breakpoint in the function after the
- // frame setup, and a compiler-generated line 0 location is not a
- // meaningful breakpoint. If none is found, return the first
- // location after the frame setup.
- if (MI.getDebugLoc().getLine())
- return std::make_pair(&MI, IsEmptyPrologue);
-
- LineZeroLoc = &MI;
- }
- IsEmptyPrologue = false;
- }
+
+ // Helper lambda to examine each instruction and potentially return it
+ // as the prologue_end point.
+ auto ExamineInst = [&](const MachineInstr &MI)
+ -> std::optional<std::pair<const MachineInstr *, bool>> {
+ // Is this instruction trivial data shuffling or frame-setup?
+ bool isCopy = (TII.isCopyInstr(MI) ? true : false);
+ bool isTrivRemat = TII.isTriviallyReMaterializable(MI);
+ bool isFrameSetup = MI.getFlag(MachineInstr::FrameSetup);
+
+ if (!isFrameSetup && MI.getDebugLoc()) {
+ // Scan forward to try to find a non-zero line number. The
+ // prologue_end marks the first breakpoint in the function after the
+ // frame setup, and a compiler-generated line 0 location is not a
+ // meaningful breakpoint. If none is found, return the first
+ // location after the frame setup.
+ if (MI.getDebugLoc().getLine())
+ return std::make_pair(&MI, IsEmptyPrologue);
+ }
+
+ // Keep track of the first "non-trivial" instruction seen, i.e. anything
+ // that doesn't involve shuffling data around or is a frame-setup.
+ if (!isCopy && !isTrivRemat && !isFrameSetup && !NonTrivialInst)
+ NonTrivialInst = &MI;
+
+ IsEmptyPrologue = false;
+ return std::nullopt;
+ };
+
+ // Examine all the instructions at the start of the function. This doesn't
+ // necessarily mean just the entry block: unoptimised code can fall-through
+ // into an initial loop, and it makes sense to put the initial breakpoint on
+ // the first instruction of such a loop. However, if we pass branches, we're
+ // better off synthesising an early prologue_end.
+ auto CurBlock = MF->begin();
+ auto CurInst = CurBlock->begin();
+ while (true) {
+ // Skip empty blocks, in rare cases the entry can be empty.
+ if (CurInst == CurBlock->end()) {
+ ++CurBlock;
+ CurInst = CurBlock->begin();
+ continue;
}
+
+ // Check whether this non-meta instruction a good position for prologue_end.
+ if (!CurInst->isMetaInstruction()) {
+ auto FoundInst = ExamineInst(*CurInst);
+ if (FoundInst)
+ return *FoundInst;
+ }
+
+ // Try to continue searching, but use a backup-location if substantive
+ // computation is happening.
+ auto NextInst = std::next(CurInst);
+ if (NextInst != CurInst->getParent()->end()) {
+ // Continue examining the current block.
+ CurInst = NextInst;
+ continue;
+ }
+
+ // We've reached the end of the block. Did we just look at a terminator?
+ if (CurInst->isTerminator()) {
+ // Some kind of "real" control flow is occurring. At the very least
+ // we would have to start exploring the CFG, a good signal that the
+ // prologue is over.
+ break;
+ }
+
+ // If we've already fallen through into a loop, don't fall through
+ // further, use a backup-location.
+ if (CurBlock->pred_size() > 1)
+ break;
+
+ // Fall-through from entry to the next block. This is common at -O0 when
+ // there's no initialisation in the function. Bail if we're also at the
+ // end of the function.
+ if (++CurBlock == MF->end())
+ break;
+ CurInst = CurBlock->begin();
+ }
+
+ // We couldn't find any source-location, suggesting all meaningful information
+ // got optimised away. Set the prologue_end to be the first non-trivial
+ // instruction, which will get the scope line number. This is better than
+ // nothing.
+ // Only do this in the entry block, as we'll be giving it the scope line for
+ // the function. Return IsEmptyPrologue==true if we've picked the first
+ // instruction.
+ if (NonTrivialInst && NonTrivialInst->getParent() == &*MF->begin()) {
+ IsEmptyPrologue = NonTrivialInst == &*MF->begin()->begin();
+ return std::make_pair(NonTrivialInst, IsEmptyPrologue);
}
- return std::make_pair(LineZeroLoc, IsEmptyPrologue);
+
+ // If the entry path is empty, just don't have a prologue_end at all.
+ return std::make_pair(nullptr, IsEmptyPrologue);
}
/// Register a source line with debug info. Returns the unique label that was
@@ -2195,12 +2282,21 @@ DwarfDebug::emitInitialLocDirective(const MachineFunction &MF, unsigned CUID) {
bool IsEmptyPrologue = PrologEnd.second;
// If the prolog is empty, no need to generate scope line for the proc.
- if (IsEmptyPrologue)
- // In degenerate cases, we can have functions with no source locations
- // at all. These want a scope line, to avoid a totally empty function.
- // Thus, only skip scope line if there's location to place prologue_end.
- if (PrologEndLoc)
- return PrologEndLoc;
+ if (IsEmptyPrologue) {
+ // If there's nowhere to put a prologue_end flag, emit a scope line in case
+ // there are simply no source locations anywhere in the function.
+ if (PrologEndLoc) {
+ // Avoid trying to assign prologue_end to a line-zero location.
+ // Instructions with no DebugLoc at all are fine, they'll be given the
+ // scope line nuumber.
+ const DebugLoc &DL = PrologEndLoc->getDebugLoc();
+ if (!DL || DL->getLine() != 0)
+ return PrologEndLoc;
+
+ // Later, don't place the prologue_end flag on this line-zero location.
+ PrologEndLoc = nullptr;
+ }
+ }
// Ensure the compile unit is created if the function is called before
// beginFunction().
diff --git a/llvm/test/CodeGen/X86/no-non-zero-debug-loc-prologue.ll b/llvm/test/CodeGen/X86/no-non-zero-debug-loc-prologue.ll
index a9c71d1d9c36b2..6ea373b67bad41 100644
--- a/llvm/test/CodeGen/X86/no-non-zero-debug-loc-prologue.ll
+++ b/llvm/test/CodeGen/X86/no-non-zero-debug-loc-prologue.ll
@@ -1,10 +1,10 @@
-; RUN: llc -filetype=asm -mtriple=x86_64-apple-macosx12.0.0 -O0 %s -o - | FileCheck %s
+; RUN: llc -filetype=asm -mtriple=x86_64-apple-macosx12.0.0 -O0 %s -o - | FileCheck %s --implicit-check-not=prologue_end
; CHECK: Lfunc_begin0:
; CHECK-NEXT: .file{{.+}}
; CHECK-NEXT: .loc 1 1 0 ## test-small.c:1:0{{$}}
; CHECK-NEXT: .cfi_startproc
; CHECK-NEXT: ## %bb.{{[0-9]+}}:
-; CHECK-NEXT: .loc 1 0 1 prologue_end{{.*}}
+; CHECK-NEXT: .loc 1 0 1
define void @test() #0 !dbg !9 {
ret void, !dbg !12
}
@@ -19,4 +19,4 @@ define void @test() #0 !dbg !9 {
!9 = distinct !DISubprogram(name: "test", scope: !10, file: !10, line: 1, type: !11, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !7)
!10 = !DIFile(filename: "test-small.c", directory: "/Users/shubham/Development/test")
!11 = !DISubroutineType(types: !7)
-!12 = !DILocation(line: 0, column: 1, scope: !9)
\ No newline at end of file
+!12 = !DILocation(line: 0, column: 1, scope: !9)
diff --git a/llvm/test/CodeGen/X86/pseudo_cmov_lower2.ll b/llvm/test/CodeGen/X86/pseudo_cmov_lower2.ll
index ee3d31bb73dca4..6091e2bf5bc56b 100644
--- a/llvm/test/CodeGen/X86/pseudo_cmov_lower2.ll
+++ b/llvm/test/CodeGen/X86/pseudo_cmov_lower2.ll
@@ -200,9 +200,9 @@ declare void @llvm.dbg.value(metadata, metadata, metadata)
; minus the DEBUG_VALUE line and changes in labels..
define double @foo1_g(float %p1, double %p2, double %p3) nounwind !dbg !4 {
; CHECK-LABEL: foo1_g:
-; CHECK: .file 1 "." "test.c"
-; CHECK-NEXT: .loc 1 3 0
; CHECK: # %bb.0: # %entry
+; CHECK-NEXT: .file 1 "." "test.c"
+; CHECK-NEXT: .loc 1 3 0 prologue_end
; CHECK-NEXT: xorps %xmm3, %xmm3
; CHECK-NEXT: ucomiss %xmm3, %xmm0
; CHECK-NEXT: movsd {{.*#+}} xmm0 = [1.25E+0,0.0E+0]
diff --git a/llvm/test/DebugInfo/MIR/X86/dbg-prologue-backup-loc2.mir b/llvm/test/DebugInfo/MIR/X86/dbg-prologue-backup-loc2.mir
new file mode 100644
index 00000000000000..c27655ac801316
--- /dev/null
+++ b/llvm/test/DebugInfo/MIR/X86/dbg-prologue-backup-loc2.mir
@@ -0,0 +1,134 @@
+# RUN: llc %s -start-before=livedebugvalues -o - | \
+# RUN: FileCheck %s --implicit-check-not=prologue_end
+#
+## When picking a "backup" location of the first non-trivial instruction in
+## a function, don't select a location outside of the entry block. We have to
+## give it the function's scope-line, and installing that outside of the entry
+## block is liable to be misleading.
+##
+## Produced from the C below with "clang -O2 -g -mllvm
+## -stop-before=livedebugvalues", then modified to unrotate and shift early
+## insts into the loop block. This means the MIR is meaningless, we only test
+## whether the scope-line will leak into the loop block or not.
+##
+## int glob = 0;
+## int foo(int arg, int sum) {
+## arg += sum;
+## while (arg) {
+## glob--;
+## arg %= glob;
+## }
+## return 0;
+## }
+#
+# CHECK-LABEL: foo:
+# CHECK: .loc 0 2 0
+# CHECK: # %bb.0:
+# CHECK-NEXT: movl %edi, %edx
+# CHECK-NEXT: .loc 0 0 0 is_stmt 0
+# CHECK-NEXT: .Ltmp0:
+# CHECK-NEXT: .p2align 4
+# CHECK-NEXT: .LBB0_1:
+# CHECK-LABEL: addl %esi, %edx
+
+
+--- |
+ ; ModuleID = 'out2.ll'
+ source_filename = "foo.c"
+ target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+ target triple = "x86_64-unknown-linux-gnu"
+
+ @glob = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0
+
+ define dso_local noundef i32 @foo(i32 noundef %arg, i32 noundef %sum) local_unnamed_addr !dbg !9 {
+ entry:
+ %add = add nsw i32 %sum, %arg
+ br label %while.body.preheader
+
+ while.body.preheader: ; preds = %entry
+ %glob.promoted = load i32, ptr @glob, align 4
+ br label %while.body, !dbg !13
+
+ while.body: ; preds = %while.body, %while.body.preheader
+ %arg.addr.06 = phi i32 [ %rem, %while.body ], [ %add, %while.body.preheader ]
+ %dec35 = phi i32 [ %dec, %while.body ], [ %glob.promoted, %while.body.preheader ]
+ %dec = add nsw i32 %dec35, -1, !dbg !14
+ %0 = add i32 %dec35, -1, !dbg !16
+ %rem = srem i32 %arg.addr.06, %0, !dbg !16
+ %tobool.not = icmp eq i32 %rem, 0, !dbg !13
+ br i1 %tobool.not, label %while.cond.while.end_crit_edge, label %while.body, !dbg !13
+
+ while.cond.while.end_crit_edge: ; preds = %while.body
+ store i32 %dec, ptr @glob, align 4, !dbg !14
+ br label %while.end, !dbg !13
+
+ while.end: ; preds = %while.cond.while.end_crit_edge
+ ret i32 0, !dbg !17
+ }
+
+ !llvm.dbg.cu = !{!2}
+ !llvm.module.flags = !{!6, !7}
+ !llvm.ident = !{!8}
+
+ !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+ !1 = distinct !DIGlobalVariable(name: "glob", scope: !2, file: !3, line: 1, type: !5, isLocal: false, isDefinition: true)
+ !2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+ !3 = !DIFile(filename: "foo.c", directory: "")
+ !4 = !{!0}
+ !5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+ !6 = !{i32 7, !"Dwarf Version", i32 5}
+ !7 = !{i32 2, !"Debug Info Version", i32 3}
+ !8 = !{!"clang"}
+ !9 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 2, type: !10, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !12)
+ !10 = !DISubroutineType(types: !11)
+ !11 = !{!5, !5, !5}
+ !12 = !{}
+ !13 = !DILocation(line: 4, column: 3, scope: !9)
+ !14 = !DILocation(line: 5, column: 9, scope: !15)
+ !15 = distinct !DILexicalBlock(scope: !9, file: !3, line: 4, column: 15)
+ !16 = !DILocation(line: 6, column: 9, scope: !15)
+ !17 = !DILocation(line: 8, column: 3, scope: !9)
+
+...
+---
+name: foo
+alignment: 16
+tracksRegLiveness: true
+debugInstrRef: true
+tracksDebugUserValues: true
+liveins:
+ - { reg: '$edi' }
+ - { reg: '$esi' }
+frameInfo:
+ maxAlignment: 1
+ maxCallFrameSize: 0
+ isCalleeSavedInfoValid: true
+machineFunctionInfo:
+ amxProgModel: None
+body: |
+ bb.0.entry:
+ liveins: $edi, $esi
+
+ $edx = MOV32rr $edi
+
+ bb.1.while.body (align 16):
+ successors: %bb.2(0x04000000), %bb.1(0x7c000000)
+ liveins: $ecx, $edx, $esi
+
+ renamable $edx = nsw ADD32rr killed renamable $edx, renamable $esi, implicit-def dead $eflags
+ renamable $ecx = MOV32rm $rip, 1, $noreg, @glob, $noreg :: (dereferenceable load (s32) from @glob)
+ renamable $ecx = DEC32r killed renamable $ecx, implicit-def dead $eflags
+ $eax = MOV32rr killed $edx
+ CDQ implicit-def $eax, implicit-def $edx, implicit $eax
+ IDIV32r renamable $ecx, implicit-def dead $eax, implicit-def $edx, implicit-def dead $eflags, implicit $eax, implicit $edx
+ TEST32rr renamable $edx, renamable $edx, implicit-def $eflags
+ JCC_1 %bb.1, 5, implicit killed $eflags
+
+ bb.2.while.cond.while.end_crit_edge:
+ liveins: $ecx, $esi
+
+ MOV32mr $rip, 1, $noreg, @glob, $noreg, killed renamable $ecx, debug-location !14 :: (store (s32) into @glob)
+ $eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags, debug-location !17
+ RET64 $eax, debug-location !17
+
+...
diff --git a/llvm/test/DebugInfo/MIR/X86/empty-inline.mir b/llvm/test/DebugInfo/MIR/X86/empty-inline.mir
index 695b7c60365b1d..9a4a5a3c5cb741 100644
--- a/llvm/test/DebugInfo/MIR/X86/empty-inline.mir
+++ b/llvm/test/DebugInfo/MIR/X86/empty-inline.mir
@@ -13,9 +13,10 @@
#
# CHECK: Address Line Column File ISA Discriminator OpIndex Flags
# CHECK-NEXT: ---
-# CHECK-NEXT: 25 0 1 0 0 0 is_stmt
-# CHECK-NEXT: 29 28 1 0 0 0 is_stmt prologue_end
+# CHECK-NEXT: 25 0 1 0 0 0 is_stmt prologue_end
+# CHECK-NEXT: 29 28 1 0 0 0 is_stmt
# CHECK-NEXT: 29 28 1 0 0 0 is_stmt end_sequence
+
--- |
source_filename = "t.ll"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
diff --git a/llvm/test/DebugInfo/X86/dbg-prolog-end-backup-loc.ll b/llvm/test/DebugInfo/X86/dbg-prolog-end-backup-loc.ll
new file mode 100644
index 00000000000000..2425df608275c5
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/dbg-prolog-end-backup-loc.ll
@@ -0,0 +1,86 @@
+; RUN: llc %s -o - | FileCheck %s
+
+;; This test has had source-locations removed from the prologue, to simulate
+;; heavily-optimised scenarios where a lot of debug-info gets dropped. Check
+;; that we can pick a "worst-case" prologue_end position, of the first
+;; instruction that does any meaningful computation (the add). It's better to
+;; put the prologue_end flag here rather than deeper into the loop, past the
+;; early-exit check.
+;;
+;; Generated from this code at -O2 -g in clang, with source locations then
+;; deleted.
+;;
+;; int glob = 0;
+;; int foo(int arg, int sum) {
+;; arg += sum;
+;; while (arg) {
+;; glob--;
+;; arg %= glob;
+;; }
+;; return 0;
+;; }
+
+; CHECK-LABEL: foo:
+;; Scope-line location:
+; CHECK: .loc 0 2 0
+;; Entry block:
+; CHECK: movl %edi, %edx
+; CHECK-NEXT: .loc 0 2 0 prologue_end
+; CHECK-NEXT: addl %esi, %edx
+; CHECK-NEXT: je .LBB0_4
+; CHECK-LABEL: # %bb.1:
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@glob = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0
+
+define dso_local noundef i32 @foo(i32 noundef %arg, i32 noundef %sum) local_unnamed_addr !dbg !9 {
+entry:
+ %add = add nsw i32 %sum, %arg
+ %tobool.not4 = icmp eq i32 %add, 0
+ br i1 %tobool.not4, label %while.end, label %while.body.preheader
+
+while.body.preheader:
+ %glob.promoted = load i32, ptr @glob, align 4
+ br label %while.body, !dbg !14
+
+while.body:
+ %arg.addr.06 = phi i32 [ %rem, %while.body ], [ %add, %while.body.preheader ]
+ %dec35 = phi i32 [ %dec, %while.body ], [ %glob.promoted, %while.body.preheader ]
+ %dec = add nsw i32 %dec35, -1, !dbg !15
+ %rem = srem i32 %arg.addr.06, %dec, !dbg !17
+ %tobool.not = icmp eq i32 %rem, 0, !dbg !14
+ br i1 %tobool.not, label %while.cond.while.end_crit_edge, label %while.body, !dbg !14
+
+while.cond.while.end_crit_edge:
+ store i32 %dec, ptr @glob, align 4, !dbg !15
+ br label %while.end, !dbg !14
+
+while.end:
+ ret i32 0, !dbg !18
+}
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!6, !7}
+!llvm.ident = !{!8}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "glob", scope: !2, file: !3, line: 1, type: !5, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "foo.c", directory: "")
+!4 = !{!0}
+!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!6 = !{i32 7, !"Dwarf Version", i32 5}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{!"clang"}
+!9 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 2, type: !10, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !12)
+!10 = !DISubroutineType(types: !11)
+!11 = !{!5, !5, !5}
+!12 = !{}
+!13 = !DILocation(line: 3, column: 7, scope: !9)
+!14 = !DILocation(line: 4, column: 3, scope: !9)
+!15 = !DILocation(line: 5, column: 9, scope: !16)
+!16 = distinct !DILexicalBlock(scope: !9, file: !3, line: 4, column: 15)
+!17 = !DILocation(line: 6, column: 9, scope: !16)
+!18 = !DILocation(line: 8, column: 3, scope: !9)
diff --git a/llvm/test/DebugInfo/X86/dbg-prolog-end.ll b/llvm/test/DebugInfo/X86/dbg-prolog-end.ll
index 1703323fc7ee1d..3d29f8e266301f 100644
--- a/llvm/test/DebugInfo/X86/dbg-prolog-end.ll
+++ b/llvm/test/DebugInfo/X86/dbg-prolog-end.ll
@@ -36,6 +36,50 @@ entry:
ret i32 %call, !dbg !16
}
+;; int foo(int arg) {
+;; while (arg)
+;; arg--;
+;; return 0;
+;; }
+;;
+;; In this function, the entry block will fall thr...
[truncated]
|
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[This is a PR to trigger the precommit CI, this commit is a reapplication of #107849, hence the skip-approval label]
In 39b2979 Pavel has kindly refined the implementation of a test in such a way that it doesn't trip up over this patch -- the test wishes to stimulate LLDBs presentation of line0 locations, rather than wanting to always step on line-zero on entry to artificial_location.c. As that's what was tripping up this change, reapply.
Original commit message follows.
[DWARF] Emit a worst-case prologue_end flag for pathological inputs (#107849)
prologue_end usually indicates where the end of the function-initialization lies, and is where debuggers usually choose to put the initial breakpoint for a function. Our current algorithm piggy-backs it on the first available source-location: which doesn't necessarily have anything to do with the start of the function.
To avoid this in heavily-optimised code that lacks many useful source locations, pick a worst-case "if all else fails" prologue_end location, of the first instruction that appears to do meaningful computation. It'll be given the function-scope line number, which should run-on from the start of the function anyway. This means if your code is completely inverted by the optimiser, you can at least put a breakpoint at the start like you expect, even if it's difficult to then step through.
This patch also attempts to preserve some good behaviour we have without optimisations -- at O0, if the prologue immediately falls into a loop body without any computation happening, then prologue_end lands at the start of that loop. This is desirable; but does mean we need to do more work to detect and support those situations.