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[NVPTX] Implement isTruncateFree(EVT FromVT, EVT ToVT) #138605

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[NVPTX] Implement isTruncateFree(EVT FromVT, EVT ToVT) #138605

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16 changes: 12 additions & 4 deletions llvm/lib/Target/NVPTX/NVPTXISelLowering.h
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Original file line number Diff line number Diff line change
Expand Up @@ -155,11 +155,19 @@ class NVPTXTargetLowering : public TargetLowering {
Instruction *I = nullptr) const override;

bool isTruncateFree(Type *SrcTy, Type *DstTy) const override {
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@kalxr kalxr May 6, 2025

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Seems like Hexagon is the only target that does it this way, but seems simpler:

llvm-project/llvm/lib/Target/Hexagon/HexagonISelLowering.cpp

Line 2145 in 1c1238d

return isTruncateFree(EVT::getEVT(Ty1), EVT::getEVT(Ty2));

// Truncating 64-bit to 32-bit is free in SASS.
if (!SrcTy->isIntegerTy() || !DstTy->isIntegerTy())
if (!(SrcTy->isIntegerTy() && DstTy->isIntegerTy()))
return false;
return SrcTy->getPrimitiveSizeInBits() == 64 &&
DstTy->getPrimitiveSizeInBits() == 32;
if (SrcTy->getPrimitiveSizeInBits() <= DstTy->getPrimitiveSizeInBits())
return false;
Comment on lines -159 to +161
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@AlexMaclean AlexMaclean May 5, 2025

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Would it be valid to call isTruncateFree if either of these conditions were not already met?

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@kalxr kalxr May 6, 2025

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I believe so. The second condition is explicitly mentioned in

llvm-project/llvm/include/llvm/CodeGen/TargetLowering.h

Line 3025 in 1c1238d

/// Targets must return false when FromTy <= ToTy.
. Most targets that override this have a check for the first condition as well.

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@AlexMaclean AlexMaclean May 6, 2025

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Interesting. What about the check for isScalarInteger? If the vector element sizes meet the criteria for being free won't the eventual expansion be free? Do we ever expect to see non integer types?

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@justinfargnoli justinfargnoli May 6, 2025 •
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Yeah, that's a good point. When expressed in PTX, the vectors become registers and thus do not guarantee contiguousness.

Notes: https://godbolt.org/z/dTo9aGEEb

return DstTy->getPrimitiveSizeInBits() % 32 == 0;
}

bool isTruncateFree(EVT FromVT, EVT ToVT) const override {
if (!(FromVT.isScalarInteger() && ToVT.isScalarInteger()))
return false;
if (FromVT.getSizeInBits() <= ToVT.getSizeInBits())
return false;
Comment on lines +166 to +169
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@AlexMaclean AlexMaclean May 5, 2025

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Same question as above.

return ToVT.getSizeInBits() % 32 == 0;
}

EVT getSetCCResultType(const DataLayout &DL, LLVMContext &Ctx,
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12 changes: 6 additions & 6 deletions llvm/test/CodeGen/NVPTX/i128-array.ll
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Expand Up @@ -8,13 +8,13 @@ define [2 x i128] @foo(i64 %a, i32 %b) {
; CHECK-NEXT: .reg .b64 %rd<5>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [foo_param_1];
; CHECK-NEXT: ld.param.u64 %rd1, [foo_param_0];
; CHECK-NEXT: shr.s64 %rd2, %rd1, 63;
; CHECK-NEXT: cvt.s64.s32 %rd3, %r1;
; CHECK-NEXT: shr.s64 %rd4, %rd3, 63;
; CHECK-NEXT: st.param.v2.b64 [func_retval0], {%rd1, %rd2};
; CHECK-NEXT: st.param.v2.b64 [func_retval0+16], {%rd3, %rd4};
; CHECK-NEXT: ld.param.s32 %rd2, [foo_param_1];
; CHECK-NEXT: cvt.u32.u64 %r1, %rd2;
; CHECK-NEXT: shr.s64 %rd3, %rd1, 63;
; CHECK-NEXT: shr.s64 %rd4, %rd2, 63;
; CHECK-NEXT: st.param.v2.b64 [func_retval0], {%rd1, %rd3};
; CHECK-NEXT: st.param.v2.b64 [func_retval0+16], {%rd2, %rd4};
; CHECK-NEXT: ret;
%1 = sext i64 %a to i128
%2 = sext i32 %b to i128
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