[MachineSink] Check predecessor/successor relationship between two basic blocks involved in critical edge splitting (#98540)

Fix an issue in #97618 - if the two basic blocks involved are not
predecessor / successor to each other, treat the candidate as illegal
for critical edge splitting.

Closes #98477 (checked in test copied from its comment).

Author: yozhu

llvm/lib/CodeGen/MachineSink.cpp

@@ -961,7 +961,7 @@ bool MachineSinking::isLegalToBreakCriticalEdge(MachineInstr &MI,
                                                 MachineBasicBlock *ToBB,
                                                 bool BreakPHIEdge) {
   // Avoid breaking back edge. From == To means backedge for single BB cycle.
-  if (!SplitEdges || FromBB == ToBB)
+  if (!SplitEdges || FromBB == ToBB || !FromBB->isSuccessor(ToBB))
     return false;
 
   MachineCycle *FromCycle = CI->getCycle(FromBB);

llvm/test/CodeGen/X86/MachineSink-Issue98477.ll

@@ -0,0 +1,76 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s | FileCheck %s
+
+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"
+
+define i32 @main(i1 %tobool.not, i32 %0) {
+; CHECK-LABEL: main:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    movl $1, %r8d
+; CHECK-NEXT:    testb $1, %dil
+; CHECK-NEXT:    jne .LBB0_8
+; CHECK-NEXT:  .LBB0_1: # %j.preheader
+; CHECK-NEXT:    xorl %r9d, %r9d
+; CHECK-NEXT:    jmp .LBB0_2
+; CHECK-NEXT:    .p2align 4, 0x90
+; CHECK-NEXT:  .LBB0_5: # %if.then4
+; CHECK-NEXT:    # in Loop: Header=BB0_2 Depth=1
+; CHECK-NEXT:    movl $1, %eax
+; CHECK-NEXT:    xorl %edx, %edx
+; CHECK-NEXT:    divl %r8d
+; CHECK-NEXT:    testb $1, %dil
+; CHECK-NEXT:    jne .LBB0_6
+; CHECK-NEXT:  .LBB0_2: # %j
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    movl $1, %eax
+; CHECK-NEXT:    xorl %edx, %edx
+; CHECK-NEXT:    idivl %esi
+; CHECK-NEXT:    movl %edx, %ecx
+; CHECK-NEXT:    testb %r9b, %r9b
+; CHECK-NEXT:    jne .LBB0_5
+; CHECK-NEXT:  # %bb.3: # %j
+; CHECK-NEXT:    # in Loop: Header=BB0_2 Depth=1
+; CHECK-NEXT:    testl %r9d, %r9d
+; CHECK-NEXT:    js .LBB0_5
+; CHECK-NEXT:  # %bb.4:
+; CHECK-NEXT:    movl %r9d, %edx
+; CHECK-NEXT:  .LBB0_6: # %if.end9
+; CHECK-NEXT:    testl %edx, %edx
+; CHECK-NEXT:    jne .LBB0_7
+; CHECK-NEXT:  .LBB0_8: # %if.end13
+; CHECK-NEXT:    xorl %r8d, %r8d
+; CHECK-NEXT:    jmp .LBB0_1
+; CHECK-NEXT:  .LBB0_7: # %while.body.lr.ph
+; CHECK-NEXT:    movl %ecx, %eax
+; CHECK-NEXT:    retq
+entry:
+  br i1 %tobool.not, label %if.end13, label %j.preheader
+
+  j.preheader:       ; preds = %if.end13, %entry
+  %h.0.ph = phi i32 [ 1, %entry ], [ 0, %if.end13 ]
+  br label %j
+
+  j:                 ; preds = %if.then4, %j.preheader
+  %1 = phi i32 [ %div2, %if.then4 ], [ 0, %j.preheader ]
+  %rem1 = srem i32 1, %0
+  %cmp = icmp slt i32 %1, 0
+  %or.cond = select i1 false, i1 true, i1 %cmp
+  br i1 %or.cond, label %if.then4, label %if.end9
+
+  if.then4:          ; preds = %j
+  %div2 = sdiv i32 1, 0
+  %rem5 = srem i32 1, %h.0.ph
+  br i1 %tobool.not, label %if.end9, label %j
+
+  if.end9:           ; preds = %if.then4, %j
+  %2 = phi i32 [ 0, %j ], [ %rem5, %if.then4 ]
+  %tobool10.not = icmp eq i32 %2, 0
+  br i1 %tobool10.not, label %if.end13, label %while.body.lr.ph
+
+  while.body.lr.ph:  ; preds = %if.end9
+  ret i32 %rem1
+
+  if.end13:          ; preds = %if.end9, %entry
+  br label %j.preheader
+}