Make use of the exact bit when optimizing '(X >>exact 3) << 1' to eliminate the

'and' that would zero out the trailing bits, and to produce an exact shift
ourselves.

llvm-svn: 147391

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/shift.ll b/llvm/test/Transforms/InstCombine/shift.ll
index 132d51a660b..0dd969b8c00 100644
--- a/llvm/test/Transforms/InstCombine/shift.ll
+++ b/llvm/test/Transforms/InstCombine/shift.ll
@@ -542,3 +542,21 @@ define i32 @test45(i32 %a) nounwind {
 ; CHECK-NEXT: %y = lshr i32 %a, 5
 ; CHECK-NEXT: ret i32 %y
 }
+
+define i32 @test46(i32 %a) {
+  %y = ashr exact i32 %a, 3
+  %z = shl i32 %y, 1
+  ret i32 %z
+; CHECK: @test46
+; CHECK-NEXT: %z = ashr exact i32 %a, 2
+; CHECK-NEXT: ret i32 %z
+}
+
+define i32 @test47(i32 %a) {
+  %y = lshr exact i32 %a, 3
+  %z = shl i32 %y, 1
+  ret i32 %z
+; CHECK: @test47
+; CHECK-NEXT: %z = lshr exact i32 %a, 2
+; CHECK-NEXT: ret i32 %z
+}