From 7866f61957c4f1f372ab5d36515b34013b7977a2 Mon Sep 17 00:00:00 2001 From: Peter Waller Date: Mon, 5 Aug 2024 16:45:52 +0100 Subject: [PATCH] [BOLT] Add --pad-funcs-before=func:n This complements --pad-funcs, and by using both simultaneously, enables moving a specific function through a binary without modifying any code other than the targeted function (and references to it) by doing (before+after=constant). See also: proposed functionality to enable inserting random padding in https://discourse.llvm.org/t/rfc-lld-feature-for-controlling-for-code-size-dependent-measurement-bias --- bolt/lib/Core/BinaryEmitter.cpp | 53 ++++++++++++++++++++++------ bolt/lib/Passes/ReorderFunctions.cpp | 12 +++++-- bolt/test/AArch64/pad-before-funcs.s | 29 +++++++++++++++ 3 files changed, 80 insertions(+), 14 deletions(-) create mode 100644 bolt/test/AArch64/pad-before-funcs.s diff --git a/bolt/lib/Core/BinaryEmitter.cpp b/bolt/lib/Core/BinaryEmitter.cpp index f34a94c577921..36f5faf8663c7 100644 --- a/bolt/lib/Core/BinaryEmitter.cpp +++ b/bolt/lib/Core/BinaryEmitter.cpp @@ -46,13 +46,17 @@ BreakFunctionNames("break-funcs", cl::Hidden, cl::cat(BoltCategory)); -static cl::list -FunctionPadSpec("pad-funcs", - cl::CommaSeparated, - cl::desc("list of functions to pad with amount of bytes"), - cl::value_desc("func1:pad1,func2:pad2,func3:pad3,..."), - cl::Hidden, - cl::cat(BoltCategory)); +cl::list + FunctionPadSpec("pad-funcs", cl::CommaSeparated, + cl::desc("list of functions to pad with amount of bytes"), + cl::value_desc("func1:pad1,func2:pad2,func3:pad3,..."), + cl::Hidden, cl::cat(BoltCategory)); + +cl::list FunctionPadBeforeSpec( + "pad-funcs-before", cl::CommaSeparated, + cl::desc("list of functions to pad with amount of bytes"), + cl::value_desc("func1:pad1,func2:pad2,func3:pad3,..."), cl::Hidden, + cl::cat(BoltCategory)); static cl::opt MarkFuncs( "mark-funcs", @@ -70,11 +74,12 @@ X86AlignBranchBoundaryHotOnly("x86-align-branch-boundary-hot-only", cl::init(true), cl::cat(BoltOptCategory)); -size_t padFunction(const BinaryFunction &Function) { +size_t padFunction(const cl::list &Spec, + const BinaryFunction &Function) { static std::map FunctionPadding; - if (FunctionPadding.empty() && !FunctionPadSpec.empty()) { - for (std::string &Spec : FunctionPadSpec) { + if (FunctionPadding.empty() && !Spec.empty()) { + for (const std::string &Spec : Spec) { size_t N = Spec.find(':'); if (N == std::string::npos) continue; @@ -319,6 +324,32 @@ bool BinaryEmitter::emitFunction(BinaryFunction &Function, Streamer.emitCodeAlignment(Function.getAlign(), &*BC.STI); } + if (size_t Padding = + opts::padFunction(opts::FunctionPadBeforeSpec, Function)) { + // Handle padFuncsBefore after the above alignment logic but before + // symbol addresses are decided. + if (!BC.HasRelocations) { + BC.errs() << "BOLT-ERROR: -pad-before-funcs is not supported in " + << "non-relocation mode\n"; + exit(1); + } + + // Preserve Function.getMinAlign(). + if (!isAligned(Function.getMinAlign(), Padding)) { + BC.errs() << "BOLT-ERROR: user-requested " << Padding + << " padding bytes before function " << Function + << " is not a multiple of the minimum function alignment (" + << Function.getMinAlign().value() << ").\n"; + exit(1); + } + + LLVM_DEBUG(dbgs() << "BOLT-DEBUG: padding before function " << Function + << " with " << Padding << " bytes\n"); + + // Since the padding is not executed, it can be null bytes. + Streamer.emitFill(Padding, 0); + } + MCContext &Context = Streamer.getContext(); const MCAsmInfo *MAI = Context.getAsmInfo(); @@ -373,7 +404,7 @@ bool BinaryEmitter::emitFunction(BinaryFunction &Function, emitFunctionBody(Function, FF, /*EmitCodeOnly=*/false); // Emit padding if requested. - if (size_t Padding = opts::padFunction(Function)) { + if (size_t Padding = opts::padFunction(opts::FunctionPadSpec, Function)) { LLVM_DEBUG(dbgs() << "BOLT-DEBUG: padding function " << Function << " with " << Padding << " bytes\n"); Streamer.emitFill(Padding, MAI->getTextAlignFillValue()); diff --git a/bolt/lib/Passes/ReorderFunctions.cpp b/bolt/lib/Passes/ReorderFunctions.cpp index 1256d71342b13..f8f6a01526dcc 100644 --- a/bolt/lib/Passes/ReorderFunctions.cpp +++ b/bolt/lib/Passes/ReorderFunctions.cpp @@ -28,7 +28,9 @@ extern cl::OptionCategory BoltOptCategory; extern cl::opt Verbosity; extern cl::opt RandomSeed; -extern size_t padFunction(const bolt::BinaryFunction &Function); +extern size_t padFunction(const cl::list &Spec, + const bolt::BinaryFunction &Function); +extern cl::list FunctionPadSpec, FunctionPadBeforeSpec; extern cl::opt ReorderFunctions; cl::opt ReorderFunctions( @@ -304,8 +306,12 @@ Error ReorderFunctions::runOnFunctions(BinaryContext &BC) { return false; if (B->isIgnored()) return true; - const size_t PadA = opts::padFunction(*A); - const size_t PadB = opts::padFunction(*B); + const size_t PadA = + opts::padFunction(opts::FunctionPadSpec, *A) + + opts::padFunction(opts::FunctionPadBeforeSpec, *A); + const size_t PadB = + opts::padFunction(opts::FunctionPadSpec, *B) + + opts::padFunction(opts::FunctionPadBeforeSpec, *B); if (!PadA || !PadB) { if (PadA) return true; diff --git a/bolt/test/AArch64/pad-before-funcs.s b/bolt/test/AArch64/pad-before-funcs.s new file mode 100644 index 0000000000000..3ce0ee5e38389 --- /dev/null +++ b/bolt/test/AArch64/pad-before-funcs.s @@ -0,0 +1,29 @@ +# Test checks that --pad-before-funcs is working as expected. +# It should be able to introduce a configurable offset for the _start symbol. +# It should reject requests which don't obey the code alignment requirement. + +# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o +# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q -Wl,--section-start=.text=0x4000 +# RUN: llvm-bolt %t.exe -o %t.bolt.0 --pad-funcs-before=_start:0 +# RUN: llvm-bolt %t.exe -o %t.bolt.4 --pad-funcs-before=_start:4 +# RUN: llvm-bolt %t.exe -o %t.bolt.8 --pad-funcs-before=_start:8 + +# RUN: not llvm-bolt %t.exe -o %t.bolt.8 --pad-funcs-before=_start:1 2>&1 | FileCheck --check-prefix=CHECK-BAD-ALIGN %s + +# CHECK-BAD-ALIGN: user-requested 1 padding bytes before function _start(*2) is not a multiple of the minimum function alignment (4). + +# RUN: llvm-objdump --section=.text --disassemble %t.bolt.0 | FileCheck --check-prefix=CHECK-0 %s +# RUN: llvm-objdump --section=.text --disassemble %t.bolt.4 | FileCheck --check-prefix=CHECK-4 %s +# RUN: llvm-objdump --section=.text --disassemble %t.bolt.8 | FileCheck --check-prefix=CHECK-8 %s + +# Trigger relocation mode in bolt. +.reloc 0, R_AARCH64_NONE + +.section .text +.globl _start + +# CHECK-0: 0000000000400000 <_start> +# CHECK-4: 0000000000400004 <_start> +# CHECK-8: 0000000000400008 <_start> +_start: + ret