[llvm] 2cd23eb - [instcombine] Fold overflow check using umulo to comparison
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Fri Jun 25 10:25:53 PDT 2021
Author: Philip Reames
Date: 2021-06-25T10:25:45-07:00
New Revision: 2cd23eb2438238b1297ff7b4368d673c449ff24f
URL: https://github.com/llvm/llvm-project/commit/2cd23eb2438238b1297ff7b4368d673c449ff24f
DIFF: https://github.com/llvm/llvm-project/commit/2cd23eb2438238b1297ff7b4368d673c449ff24f.diff
LOG: [instcombine] Fold overflow check using umulo to comparison
If we have a umul.with.overflow where the multiply result is not used and one of the operands is a constant, we can perform the overflow check cheaper with a comparison then by performing the multiply and extracting the overflow flag.
(Noticed when looking at the conditions SCEV emits for overflow checks.)
Differential Revision: https://reviews.llvm.org/D104665
Added:
Modified:
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
llvm/test/Transforms/InstCombine/umulo.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 67aca4e345add..2dee94392c218 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3083,13 +3083,36 @@ Instruction *InstCombinerImpl::visitExtractValueInst(ExtractValueInst &EV) {
return BinaryOperator::Create(BinOp, LHS, RHS);
}
- // If the normal result of the add is dead, and the RHS is a constant,
- // we can transform this into a range comparison.
- // overflow = uadd a, -4 --> overflow = icmp ugt a, 3
- if (WO->getIntrinsicID() == Intrinsic::uadd_with_overflow)
+ assert(*EV.idx_begin() == 1 &&
+ "unexpected extract index for overflow inst");
+
+ // If the normal result of the computation is dead, and the RHS is a
+ // constant, we can transform this into a range comparison for many cases.
+ // TODO: We can generalize these for non-constant rhs when the newly
+ // formed expressions are known to simplify. Constants are merely one
+ // such case.
+ // TODO: Handle vector splats.
+ switch (WO->getIntrinsicID()) {
+ default:
+ break;
+ case Intrinsic::uadd_with_overflow:
+ // overflow = uadd a, -4 --> overflow = icmp ugt a, 3
if (ConstantInt *CI = dyn_cast<ConstantInt>(WO->getRHS()))
return new ICmpInst(ICmpInst::ICMP_UGT, WO->getLHS(),
ConstantExpr::getNot(CI));
+ break;
+ case Intrinsic::umul_with_overflow:
+ // overflow for umul a, C --> a > UINT_MAX udiv C
+ // (unless C == 0, in which case no overflow ever occurs)
+ if (ConstantInt *CI = dyn_cast<ConstantInt>(WO->getRHS())) {
+ assert(!CI->isZero() && "handled by instruction simplify");
+ auto UMax = APInt::getMaxValue(CI->getType()->getBitWidth());
+ auto *Op =
+ ConstantExpr::getUDiv(ConstantInt::get(CI->getType(), UMax), CI);
+ return new ICmpInst(ICmpInst::ICMP_UGT, WO->getLHS(), Op);
+ }
+ break;
+ };
}
}
if (LoadInst *L = dyn_cast<LoadInst>(Agg))
diff --git a/llvm/test/Transforms/InstCombine/umulo.ll b/llvm/test/Transforms/InstCombine/umulo.ll
index cce455e842881..3fb0b9443a904 100644
--- a/llvm/test/Transforms/InstCombine/umulo.ll
+++ b/llvm/test/Transforms/InstCombine/umulo.ll
@@ -35,8 +35,7 @@ define i1 @test_constant1(i8 %a) {
define i1 @test_constant2(i8 %a) {
; CHECK-LABEL: @test_constant2(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 2)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp slt i8 [[A:%.*]], 0
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 2)
@@ -46,8 +45,7 @@ define i1 @test_constant2(i8 %a) {
define i1 @test_constant3(i8 %a) {
; CHECK-LABEL: @test_constant3(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 3)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp ugt i8 [[A:%.*]], 85
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 3)
@@ -57,8 +55,7 @@ define i1 @test_constant3(i8 %a) {
define i1 @test_constant4(i8 %a) {
; CHECK-LABEL: @test_constant4(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 4)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp ugt i8 [[A:%.*]], 63
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 4)
@@ -69,8 +66,7 @@ define i1 @test_constant4(i8 %a) {
define i1 @test_constant127(i8 %a) {
; CHECK-LABEL: @test_constant127(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 127)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp ugt i8 [[A:%.*]], 2
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 127)
@@ -80,8 +76,7 @@ define i1 @test_constant127(i8 %a) {
define i1 @test_constant128(i8 %a) {
; CHECK-LABEL: @test_constant128(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 -128)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp ugt i8 [[A:%.*]], 1
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 128)
@@ -91,8 +86,7 @@ define i1 @test_constant128(i8 %a) {
define i1 @test_constant255(i8 %a) {
; CHECK-LABEL: @test_constant255(
-; CHECK-NEXT: [[RES:%.*]] = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 [[A:%.*]], i8 -1)
-; CHECK-NEXT: [[OVERFLOW:%.*]] = extractvalue { i8, i1 } [[RES]], 1
+; CHECK-NEXT: [[OVERFLOW:%.*]] = icmp ugt i8 [[A:%.*]], 1
; CHECK-NEXT: ret i1 [[OVERFLOW]]
;
%res = tail call { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 255)
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