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Differential D60641

[DebugInfo@O2] Prevent Instcombine from dropping debug info when removing zexts (fixes PR41475)
ClosedPublic

Authored by wolfgangp on Apr 12 2019, 3:41 PM.

Details

Reviewers
vsk
aprantl
jmorse
probinson
Commits
rG4fe42214e242: [DEBUGINFO] Prevent Instcombine from dropping debuginfo when removing zexts
rL358431: [DEBUGINFO] Prevent Instcombine from dropping debuginfo when removing zexts
Summary

See PR41475 for a short description of the problem.

salvageDebugInfo() in Transforms/Utils/Local.cpp decides how an instruction's actions are affecting a value that is used in a dbg.value() call and attempts to return a DIExpr that would make it possible to reconstruct the original value from the instruction's source operand in case the instruction is removed by someone (e.g. by instcombine). Noop casts don't change the value and we can safely say that the cast's source operand can be used by the debug user, thus we simply return the DIExpr found there.

This patch extends this to zext instructions, which are not a noop cast, but arguably don't change the value. Thus the source value of a zext can replace the result value in debug users and survive a removal of the zext instruction.

Diff Detail

Repository
rL LLVM

Event Timeline

wolfgangp created this revision.Apr 12 2019, 3:41 PM
aprantl accepted this revision.Apr 12 2019, 3:45 PM
This revision is now accepted and ready to land.Apr 12 2019, 3:45 PM
Closed by commit rL358431: [DEBUGINFO] Prevent Instcombine from dropping debuginfo when removing zexts (authored by wolfgangp). · Explain WhyApr 15 2019, 10:40 AM
This revision was automatically updated to reflect the committed changes.
Herald added a project: Restricted Project. · View Herald TranscriptApr 15 2019, 10:40 AM

Revision Contents

PathSize
llvm/
trunk/
lib/
Transforms/
Utils/
9 lines
test/
Transforms/
InstCombine/
9 lines

Diff 195226

llvm/trunk/lib/Transforms/Utils/Local.cpp

Show First 20 Lines • Show All 1,660 Lines • ▼ Show 20 LinesDIExpression *llvm::salvageDebugInfoImpl(Instruction &I,
// initializer-list helper for applying operators to the source DIExpression. // initializer-list helper for applying operators to the source DIExpression.
auto applyOps = auto applyOps =
[&](std::initializer_list<uint64_t> Opcodes) -> DIExpression * { [&](std::initializer_list<uint64_t> Opcodes) -> DIExpression * {
SmallVector<uint64_t, 8> Ops(Opcodes); SmallVector<uint64_t, 8> Ops(Opcodes);
return doSalvage(Ops); return doSalvage(Ops);
}; };
if (auto *CI = dyn_cast<CastInst>(&I)) { if (auto *CI = dyn_cast<CastInst>(&I)) {
if (!CI->isNoopCast(DL)) // No-op casts and zexts are irrelevant for debug info.
return nullptr; if (CI->isNoopCast(DL) || isa<ZExtInst>(&I))
// No-op casts are irrelevant for debug info.
return SrcDIExpr; return SrcDIExpr;
return nullptr;
} else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) { } else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
unsigned BitWidth = unsigned BitWidth =
M.getDataLayout().getIndexSizeInBits(GEP->getPointerAddressSpace()); M.getDataLayout().getIndexSizeInBits(GEP->getPointerAddressSpace());
// Rewrite a constant GEP into a DIExpression. // Rewrite a constant GEP into a DIExpression.
APInt Offset(BitWidth, 0); APInt Offset(BitWidth, 0);
if (GEP->accumulateConstantOffset(M.getDataLayout(), Offset)) { if (GEP->accumulateConstantOffset(M.getDataLayout(), Offset)) {
return applyOffset(Offset.getSExtValue()); return applyOffset(Offset.getSExtValue());
} else { } else {
▲ Show 20 Lines • Show All 1,245 Lines • Show Last 20 Lines

llvm/trunk/test/Transforms/InstCombine/cast-mul-select.ll

Show First 20 Lines • Show All 164 Lines • ▼ Show 20 Lines
exit2: exit2:
unreachable unreachable
exit: exit:
unreachable unreachable
} }
; Check that we don't drop debug info when a zext is removed.
define i1 @foo(i1 zeroext %b) {
; DBGINFO-LABEL: @foo(
; DBGINFO-NEXT: call void @llvm.dbg.value(metadata i1 %b
; DBGINFO-NEXT: ret i1 %b
%frombool = zext i1 %b to i8
ret i1 %b
}