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[PGO] Skip if an IndirectBrInst critical edge cannot be split

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PGOInstrumentation runs `SplitIndirectBrCriticalEdges` but some IndirectBrInst
critical edge cannot be split. `getInstrBB` will crash when calling `SplitCriticalEdge`, e.g.

  int foo(char *p) {
    void *targets[2];
    targets[0] = &&indirect;
    targets[1] = &&end;
    for (;; p++)
      if (*p == 7) {
  indirect:
        goto *targets[p[1]]; // the self loop is critical in -O
      }
  end:
    return 0;
  }

Skip such critical edges to prevent a crash.

Reviewed By: davidxl, lebedev.ri

Differential Revision: https://reviews.llvm.org/D87435
This commit is contained in:
Fangrui Song 2020-09-10 11:03:48 -07:00
parent 12a4a51f26
commit a388aec5dc
2 changed files with 28 additions and 1 deletions
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5
lib/Transforms/Instrumentation/PGOInstrumentation.cpp
View File

@ -807,8 +807,11 @@ BasicBlock *FuncPGOInstrumentation<Edge, BBInfo>::getInstrBB(Edge *E) {
if (!E->IsCritical)
return canInstrument(DestBB);
// Some IndirectBr critical edges cannot be split by the previous
// SplitIndirectBrCriticalEdges call. Bail out.
unsigned SuccNum = GetSuccessorNumber(SrcBB, DestBB);
BasicBlock *InstrBB = SplitCriticalEdge(TI, SuccNum);
BasicBlock *InstrBB =
isa<IndirectBrInst>(TI) ? nullptr : SplitCriticalEdge(TI, SuccNum);
if (!InstrBB) {
LLVM_DEBUG(
dbgs() << "Fail to split critical edge: not instrument this edge.\n");

24
test/Transforms/PGOProfile/split-indirectbr-critical-edges.ll
View File

@ -37,3 +37,27 @@ if.end: ; preds = %if.end.preheader, %
indirectbr i8* %2, [label %for.cond2, label %if.end]
; CHECK: indirectbr i8* %2, [label %for.cond2, label %if.end]
}
;; If an indirectbr critical edge cannot be split, ignore it.
;; The edge will not be profiled.
; CHECK-LABEL: @cannot_split(
; CHECK-NEXT: entry:
; CHECK-NEXT: call void @llvm.instrprof.increment
; CHECK-NOT: call void @llvm.instrprof.increment
define i32 @cannot_split(i8* nocapture readonly %p) {
entry:
%targets = alloca <2 x i8*>, align 16
store <2 x i8*> <i8* blockaddress(@cannot_split, %indirect), i8* blockaddress(@cannot_split, %end)>, <2 x i8*>* %targets, align 16
%arrayidx2 = getelementptr inbounds i8, i8* %p, i64 1
%0 = load i8, i8* %arrayidx2
%idxprom = sext i8 %0 to i64
%arrayidx3 = getelementptr inbounds <2 x i8*>, <2 x i8*>* %targets, i64 0, i64 %idxprom
%1 = load i8*, i8** %arrayidx3, align 8
br label %indirect
indirect: ; preds = %entry, %indirect
indirectbr i8* %1, [label %indirect, label %end]
end: ; preds = %indirect
ret i32 0
}