Transform any logical shift of a power of two into an exact/NUW shift when
in a known-non-zero context.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131887 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index fdec640..f3d1061 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -29,6 +29,23 @@
// code.
if (!V->hasOneUse()) return 0;
+
+ // (PowerOfTwo >>u B) --> isExact since shifting out the result would make it
+ // inexact. Similarly for <<.
+ if (BinaryOperator *I = dyn_cast<BinaryOperator>(V))
+ if (I->isLogicalShift() &&
+ isPowerOfTwo(I->getOperand(0), IC.getTargetData())) {
+ if (I->getOpcode() == Instruction::LShr && !I->isExact()) {
+ I->setIsExact();
+ return I;
+ }
+
+ if (I->getOpcode() == Instruction::Shl && !I->hasNoUnsignedWrap()) {
+ I->setHasNoUnsignedWrap();
+ return I;
+ }
+ }
+
// ((1 << A) >>u B) --> (1 << (A-B))
// Because V cannot be zero, we know that B is less than A.
Value *A = 0, *B = 0, *PowerOf2 = 0;
diff --git a/test/Transforms/InstCombine/shift.ll b/test/Transforms/InstCombine/shift.ll
index bded68a..d9ac9cb 100644
--- a/test/Transforms/InstCombine/shift.ll
+++ b/test/Transforms/InstCombine/shift.ll
@@ -506,3 +506,24 @@
; CHECK-NEXT: shl i32 8, %b
; CHECK-NEXT: ret i32
}
+
+define i32 @test42(i32 %a, i32 %b) nounwind {
+ %div = lshr i32 4096, %b ; must be exact otherwise we'd divide by zero
+ %div2 = udiv i32 %a, %div
+ ret i32 %div2
+; CHECK: @test42
+; CHECK-NEXT: lshr exact i32 4096, %b
+}
+
+define i32 @test43(i32 %a, i32 %b) nounwind {
+ %div = shl i32 4096, %b ; must be exact otherwise we'd divide by zero
+ %div2 = udiv i32 %a, %div
+ ret i32 %div2
+; CHECK: @test43
+; CHECK-NEXT: add i32 %b, 12
+; CHECK-NEXT: lshr
+; CHECK-NEXT: ret
+}
+
+
+