InstCombine: Annotate sub with nsw when we prove it's safe

We can prove that a 'sub' can be a 'sub nsw' under certain conditions:
- The sign bits of the operands is the same.
- Both operands have more than 1 sign bit.

The subtraction cannot be a signed overflow in either case.

llvm-svn: 216037

1 files changed, 36 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index 114aff7efb8..0d310a8deeb 100644
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -473,3 +473,39 @@ define i32 @test39(i32 %A, i32 %x) {
 ; CHECK: %C = add i32 %x, %A
 ; CHECK: ret i32 %C
 }
+
+define i16 @test40(i16 %a, i16 %b) {
+  %ashr = ashr i16 %a, 1
+  %ashr1 = ashr i16 %b, 1
+  %sub = sub i16 %ashr, %ashr1
+  ret i16 %sub
+; CHECK-LABEL: @test40(
+; CHECK-NEXT: [[ASHR:%.*]] = ashr i16 %a, 1
+; CHECK-NEXT: [[ASHR1:%.*]] = ashr i16 %b, 1
+; CHECK-NEXT: [[RET:%.*]] = sub nsw i16 [[ASHR]], [[ASHR1]]
+; CHECK: ret i16 [[RET]]
+}
+
+define i32 @test41(i16 %a, i16 %b) {
+  %conv = sext i16 %a to i32
+  %conv1 = sext i16 %b to i32
+  %sub = sub i32 %conv, %conv1
+  ret i32 %sub
+; CHECK-LABEL: @test41(
+; CHECK-NEXT: [[SEXT:%.*]] = sext i16 %a to i32
+; CHECK-NEXT: [[SEXT1:%.*]] = sext i16 %b to i32
+; CHECK-NEXT: [[RET:%.*]] = sub nsw i32 [[SEXT]], [[SEXT1]]
+; CHECK: ret i32 [[RET]]
+}
+
+define i4 @test42(i4 %x, i4 %y) {
+  %a = and i4 %y, 7
+  %b = and i4 %x, 7
+  %c = sub i4 %a, %b
+  ret i4 %c
+; CHECK-LABEL: @test42(
+; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
+; CHECK-NEXT: [[AND1:%.*]] = and i4 %x, 7
+; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 %a, %b
+; CHECK: ret i4 [[RET]]
+}