InstCombine: Don't miscompile X % ((Pow2 << A) >>u B)

We assumed that A must be greater than B because the right hand side of
a remainder operator must be nonzero.

However, it is possible for A to be less than B if Pow2 is a power of
two greater than 1.

Take for example:
i32 %A = 0
i32 %B = 31
i32 Pow2 = 2147483648

((Pow2 << 0) >>u 31) is non-zero but A is less than B.

This fixes PR21274.

llvm-svn: 219713

1 files changed, 13 insertions, 1 deletions

diff --git a/llvm/test/Transforms/InstCombine/div.ll b/llvm/test/Transforms/InstCombine/div.ll
index f2a70fd0f0d..284104317d4 100644
--- a/llvm/test/Transforms/InstCombine/div.ll
+++ b/llvm/test/Transforms/InstCombine/div.ll
@@ -265,7 +265,7 @@ define i32 @test30(i32 %a) {
 ; CHECK-NEXT: ret i32 %a
 }
 
-define <2 x i32> @test31(<2 x i32> %x) nounwind {
+define <2 x i32> @test31(<2 x i32> %x) {
   %shr = lshr <2 x i32> %x, <i32 31, i32 31>
   %div = udiv <2 x i32> %shr, <i32 2147483647, i32 2147483647>
   ret <2 x i32> %div
@@ -274,3 +274,15 @@ define <2 x i32> @test31(<2 x i32> %x) nounwind {
 ; CHECK-NEXT: udiv <2 x i32> %[[shr]], <i32 2147483647, i32 2147483647>
 ; CHECK-NEXT: ret <2 x i32>
 }
+
+define i32 @test32(i32 %a, i32 %b) {
+  %shl = shl i32 2, %b
+  %div = lshr i32 %shl, 2
+  %div2 = udiv i32 %a, %div
+  ret i32 %div2
+; CHECK-LABEL: @test32(
+; CHECK-NEXT: %[[shl:.*]] = shl i32 2, %b
+; CHECK-NEXT: %[[shr:.*]] = lshr i32 %[[shl]], 2
+; CHECK-NEXT: %[[div:.*]] = udiv i32 %a, %[[shr]]
+; CHECK-NEXT: ret i32
+}