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[SelectionDAG] Mark frame index as "aliased" at argument copy elison #89712

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Merged
merged 1 commit into from
Apr 23, 2024
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[SelectionDAG] Mark frame index as "aliased" at argument copy elison #89712

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7 changes: 7 additions & 0 deletions llvm/include/llvm/CodeGen/MachineFrameInfo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -701,6 +701,13 @@ class MachineFrameInfo {
return Objects[ObjectIdx+NumFixedObjects].isAliased;
}

/// Set "maybe pointed to by an LLVM IR value" for an object.
void setIsAliasedObjectIndex(int ObjectIdx, bool IsAliased) {
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Suggested change
void setIsAliasedObjectIndex(int ObjectIdx, bool IsAliased) {
void setIsAliasedObjectIndex(int ObjectIdx, bool IsAliased = true) {

assert(unsigned(ObjectIdx+NumFixedObjects) < Objects.size() &&
"Invalid Object Idx!");
Objects[ObjectIdx+NumFixedObjects].isAliased = IsAliased;
}

/// Returns true if the specified index corresponds to an immutable object.
bool isImmutableObjectIndex(int ObjectIdx) const {
// Tail calling functions can clobber their function arguments.
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3 changes: 2 additions & 1 deletion llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
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Expand Up @@ -11128,14 +11128,15 @@ static void tryToElideArgumentCopy(
}

// Perform the elision. Delete the old stack object and replace its only use
// in the variable info map. Mark the stack object as mutable.
// in the variable info map. Mark the stack object as mutable and aliased.
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Is there any practical difference between the mutable and isAliased cases? Where is the ultimate alias query that goes wrong?

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isAliased is used inside ScheduleDAGInstrs.cpp getUnderlyingObjectsForInstr:

      if (const PseudoSourceValue *PSV = MMO->getPseudoValue()) {
        // Function that contain tail calls don't have unique PseudoSourceValue
        // objects. Two PseudoSourceValues might refer to the same or
        // overlapping locations. The client code calling this function assumes
        // this is not the case. So return a conservative answer of no known
        // object.
        if (MFI.hasTailCall())
          return false;

        // For now, ignore PseudoSourceValues which may alias LLVM IR values
        // because the code that uses this function has no way to cope with
        // such aliases.
        if (PSV->isAliased(&MFI))
          return false;

        bool MayAlias = PSV->mayAlias(&MFI);
        Objects.emplace_back(PSV, MayAlias);
      } else if (const Value *V = MMO->getValue()) {

LLVM_DEBUG({
dbgs() << "Eliding argument copy from " << Arg << " to " << *AI << '\n'
<< " Replacing frame index " << OldIndex << " with " << FixedIndex
<< '\n';
});
MFI.RemoveStackObject(OldIndex);
MFI.setIsImmutableObjectIndex(FixedIndex, false);
MFI.setIsAliasedObjectIndex(FixedIndex, true);
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Suggested change
MFI.setIsAliasedObjectIndex(FixedIndex, true);
MFI.setIsAliasedObjectIndex(FixedIndex);

AllocaIndex = FixedIndex;
ArgCopyElisionFrameIndexMap.insert({OldIndex, FixedIndex});
for (SDValue ArgVal : ArgVals)
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6 changes: 2 additions & 4 deletions llvm/test/CodeGen/Hexagon/arg-copy-elison.ll
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Expand Up @@ -18,14 +18,12 @@ define i32 @f(i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: {
; CHECK-NEXT: r0 = memw(r29+#36)
; CHECK-NEXT: memw(r29+#32) = ##666
; CHECK-NEXT: }
; CHECK-NEXT: {
; CHECK-NEXT: r1 = memw(r29+#28)
; CHECK-NEXT: r2 = memw(r29+#32)
; CHECK-NEXT: }
; CHECK-NEXT: {
; CHECK-NEXT: r0 = sub(r1,r0)
; CHECK-NEXT: r2 = memw(r29+#32)
; CHECK-NEXT: memw(r29+#32) = ##666
; CHECK-NEXT: }
; CHECK-NEXT: {
; CHECK-NEXT: r0 = xor(r0,r2)
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