From b70ebd2aa3b6f4546d4734e7bcdbed2017036b4d Mon Sep 17 00:00:00 2001
From: Benjamin Kramer <benny.kra@googlemail.com>
Date: Tue, 23 Nov 2010 18:52:42 +0000
Subject: [PATCH] InstCombine: Reduce "X shift (A srem B)" to "X shift (A urem
B)" iff B is positive.
This allows to transform the rem in "1 << ((int)x % 8);" to an and.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120028 91177308-0d34-0410-b5e6-96231b3b80d8
---
lib/Target/README.txt | 12 ++----------
lib/Transforms/InstCombine/InstCombineShifts.cpp | 15 +++++++++++++++
test/Transforms/InstCombine/shift.ll | 11 +++++++++++
3 files changed, 28 insertions(+), 10 deletions(-)
diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 4d5b17ab367..b98531ca859 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1698,22 +1698,14 @@ from gcc.
//===---------------------------------------------------------------------===//
Missed instcombine transformation:
-define i32 @a(i32 %x) nounwind readnone {
-entry:
- %rem = srem i32 %x, 32
- %shl = shl i32 1, %rem
- ret i32 %shl
-}
-
-The srem can be transformed to an and because if x is negative, the shift is
-undefined. Here's a more general case that occurs on 64-bit targets:
%382 = srem i32 %tmp14.i, 64 ; [#uses=1]
%383 = zext i32 %382 to i64 ; [#uses=1]
%384 = shl i64 %381, %383 ; [#uses=1]
%385 = icmp slt i32 %tmp14.i, 64 ; [#uses=1]
-Testcase derived from 403.gcc.
+The srem can be transformed to an and because if x is negative, the shift is
+undefined. Testcase derived from 403.gcc.
//===---------------------------------------------------------------------===//
diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 9f7d98ed794..7969dbfec04 100644
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -53,6 +53,21 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
if (ConstantInt *CUI = dyn_cast<ConstantInt>(Op1))
if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
return Res;
+
+ // X shift (A srem B) -> X shift (A urem B) iff B is positive.
+ // Because shifts by negative values are undefined.
+ if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
+ if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
+ // Make sure the divisor's sign bit is zero.
+ APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
+ if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
+ Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
+ BO->getName());
+ I.setOperand(1, URem);
+ return &I;
+ }
+ }
+
return 0;
}
diff --git a/test/Transforms/InstCombine/shift.ll b/test/Transforms/InstCombine/shift.ll
index 871e9fe070e..6bebca9cc85 100644
--- a/test/Transforms/InstCombine/shift.ll
+++ b/test/Transforms/InstCombine/shift.ll
@@ -441,3 +441,14 @@ entry:
; CHECK: %ins = or i128 %tmp23, %A
; CHECK: %tmp46 = trunc i128 %ins to i64
}
+
+define i32 @test38(i32 %x) nounwind readnone {
+entry:
+ %rem = srem i32 %x, 32
+ %shl = shl i32 1, %rem
+ ret i32 %shl
+; CHECK: @test38
+; CHECK-NOT: srem
+; CHECK: ret i32
+}
+