[llvm] [RISCV] Support constant hoisting of immediate store values (PR #96073)
Alex Bradbury via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 19 06:56:13 PDT 2024
https://github.com/asb created https://github.com/llvm/llvm-project/pull/96073
Previously getIntImmInstCost only calculated the cost of materialising the argument of a store if it was the address. This means ConstantHoisting's transformation wouldn't kick in for cases like storing two values that require multiple instructions to materialise but where one can be cheaply generated from the other (e.g. by an addition).
Two key changes were needed to avoid regressions when enabling this:
* Allowing constant materialisation cost to be calculated assuming zeroes are free (as might happen if you had a 2*XLEN constant and one half is zero).
* Avoiding constant hoisting if we have a misaligned store that's going to be a legalised to a sequence of narrower stores. I'm seeing cases where hoisting the constant ends up with worse codegen in that case.
>From e8715d257965a2e09e4b82d5a5ad63f6d6267742 Mon Sep 17 00:00:00 2001
From: Alex Bradbury <asb at igalia.com>
Date: Wed, 19 Jun 2024 14:52:53 +0100
Subject: [PATCH] [RISCV] Support constant hoisting of immediate store values
Previously getIntImmInstCost only calculated the cost of materialising
the argument of a store if it was the address. This means
ConstantHoisting's transformation wouldn't kick in for cases like
storing two values that require multiple instructions to materialise but
where one can be cheaply generated from the other (e.g. by an addition).
---
.../Target/RISCV/MCTargetDesc/RISCVMatInt.cpp | 6 +++--
.../Target/RISCV/MCTargetDesc/RISCVMatInt.h | 7 +++++-
.../Target/RISCV/RISCVTargetTransformInfo.cpp | 23 +++++++++++++++----
llvm/test/CodeGen/RISCV/pr64935.ll | 5 +---
.../ConstantHoisting/RISCV/immediates.ll | 7 +++---
5 files changed, 33 insertions(+), 15 deletions(-)
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp
index 0a857eb96935e..3c927b7413136 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp
@@ -499,7 +499,7 @@ InstSeq generateTwoRegInstSeq(int64_t Val, const MCSubtargetInfo &STI,
}
int getIntMatCost(const APInt &Val, unsigned Size, const MCSubtargetInfo &STI,
- bool CompressionCost) {
+ bool CompressionCost, bool FreeZeroes) {
bool IsRV64 = STI.hasFeature(RISCV::Feature64Bit);
bool HasRVC = CompressionCost && (STI.hasFeature(RISCV::FeatureStdExtC) ||
STI.hasFeature(RISCV::FeatureStdExtZca));
@@ -510,10 +510,12 @@ int getIntMatCost(const APInt &Val, unsigned Size, const MCSubtargetInfo &STI,
int Cost = 0;
for (unsigned ShiftVal = 0; ShiftVal < Size; ShiftVal += PlatRegSize) {
APInt Chunk = Val.ashr(ShiftVal).sextOrTrunc(PlatRegSize);
+ if (FreeZeroes && Chunk == 0)
+ continue;
InstSeq MatSeq = generateInstSeq(Chunk.getSExtValue(), STI);
Cost += getInstSeqCost(MatSeq, HasRVC);
}
- return std::max(1, Cost);
+ return std::max(FreeZeroes ? 0 : 1, Cost);
}
OpndKind Inst::getOpndKind() const {
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.h b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.h
index e87e0f3256470..ae94f3778b217 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.h
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.h
@@ -71,8 +71,13 @@ InstSeq generateTwoRegInstSeq(int64_t Val, const MCSubtargetInfo &STI,
// If CompressionCost is true it will use a different cost calculation if RVC is
// enabled. This should be used to compare two different sequences to determine
// which is more compressible.
+//
+// If FreeZeroes is true, it will be assumed free to materialize any
+// XLen-sized chunks that are 0. This is appropriate to use in instances when
+// the zero register can be used, e.g. when estimating the cost of
+// materializing a value used by a particular operation.
int getIntMatCost(const APInt &Val, unsigned Size, const MCSubtargetInfo &STI,
- bool CompressionCost = false);
+ bool CompressionCost = false, bool FreeZeroes = false);
} // namespace RISCVMatInt
} // namespace llvm
#endif
diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
index 176d0e79253ac..17b3ce01f3cc8 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
@@ -120,7 +120,9 @@ InstructionCost RISCVTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty,
// Otherwise, we check how many instructions it will take to materialise.
const DataLayout &DL = getDataLayout();
- return RISCVMatInt::getIntMatCost(Imm, DL.getTypeSizeInBits(Ty), *getST());
+ return RISCVMatInt::getIntMatCost(Imm, DL.getTypeSizeInBits(Ty), *getST(),
+ /*CompressionCost=*/false,
+ /*FreeZeroes=*/true);
}
// Look for patterns of shift followed by AND that can be turned into a pair of
@@ -172,11 +174,22 @@ InstructionCost RISCVTTIImpl::getIntImmCostInst(unsigned Opcode, unsigned Idx,
// split up large offsets in GEP into better parts than ConstantHoisting
// can.
return TTI::TCC_Free;
- case Instruction::Store:
- // If the address is a constant, use the materialization cost.
- if (Idx == 1)
+ case Instruction::Store: {
+ // Use the materialization cost regardless of if it's the address or the
+ // value that is constant, except for if the store is misaligned and
+ // misaligned accesses are not legal (experience shows constant hoisting
+ // can sometimes be harmful in such cases).
+ if (Idx == 1 || !Inst)
return getIntImmCost(Imm, Ty, CostKind);
- return TTI::TCC_Free;
+
+ StoreInst *ST = cast<StoreInst>(Inst);
+ if (!getTLI()->allowsMemoryAccessForAlignment(
+ Ty->getContext(), DL, getTLI()->getValueType(DL, Ty),
+ ST->getPointerAddressSpace(), ST->getAlign()))
+ return TTI::TCC_Free;
+
+ return getIntImmCost(Imm, Ty, CostKind);
+ }
case Instruction::Load:
// If the address is a constant, use the materialization cost.
return getIntImmCost(Imm, Ty, CostKind);
diff --git a/llvm/test/CodeGen/RISCV/pr64935.ll b/llvm/test/CodeGen/RISCV/pr64935.ll
index 60be5fa6c994e..b712db0dc99d6 100644
--- a/llvm/test/CodeGen/RISCV/pr64935.ll
+++ b/llvm/test/CodeGen/RISCV/pr64935.ll
@@ -4,10 +4,7 @@
define i1 @f() {
; CHECK-LABEL: f:
; CHECK: # %bb.0:
-; CHECK-NEXT: lui a0, 524288
-; CHECK-NEXT: not a0, a0
-; CHECK-NEXT: sltiu a0, a0, 2
-; CHECK-NEXT: xori a0, a0, 1
+; CHECK-NEXT: li a0, 1
; CHECK-NEXT: ret
%B25 = shl i64 4294967296, -9223372036854775808
%B13 = sub i64 -1, -9223372036854775808
diff --git a/llvm/test/Transforms/ConstantHoisting/RISCV/immediates.ll b/llvm/test/Transforms/ConstantHoisting/RISCV/immediates.ll
index 8f57df6edb2c0..29df749e10059 100644
--- a/llvm/test/Transforms/ConstantHoisting/RISCV/immediates.ll
+++ b/llvm/test/Transforms/ConstantHoisting/RISCV/immediates.ll
@@ -211,11 +211,12 @@ exit:
; Check that we use a common base for immediates needed by a store if the
; constants require more than 1 instruction.
-; TODO: This doesn't trigger currently.
define void @test20(ptr %p1, ptr %p2) {
; CHECK-LABEL: test20
-; CHECK: store i32 15111111, ptr %p1
-; CHECK: store i32 15111112, ptr %p2
+; CHECK: %const = bitcast i32 15111111 to i32
+; CHECK: store i32 %const, ptr %p1, align 4
+; CHECK: %const_mat = add i32 %const, 1
+; CHECK: store i32 %const_mat, ptr %p2, align 4
store i32 15111111, ptr %p1, align 4
store i32 15111112, ptr %p2, align 4
ret void
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