[llvm] [InstCombine] Fold ((X << nuw Z) + nuw Y) >>u Z --> X + nuw (Y >>u Z) (PR #88193)

[Nikita Popov via llvm-commits]

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@@ -360,8 +371,57 @@ define <3 x i14> @mul_splat_fold_vec(<3 x i14> %x) {
   ret <3 x i14> %t
 }
 
+define i32 @shl_add_lshr_flag_preservation(i32 %x, i32 %c, i32 %y) {
+; CHECK-LABEL: @shl_add_lshr_flag_preservation(
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr exact i32 [[Y:%.*]], [[C:%.*]]
+; CHECK-NEXT:    [[LSHR:%.*]] = add nuw nsw i32 [[TMP1]], [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[LSHR]]
+;
+  %shl = shl nuw i32 %x, %c
+  %add = add nuw nsw i32 %shl, %y
+  %lshr = lshr exact i32 %add, %c
+  ret i32 %lshr
+}
+
+define i32 @shl_add_lshr(i32 %x, i32 %c, i32 %y) {
+; CHECK-LABEL: @shl_add_lshr(
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[Y:%.*]], [[C:%.*]]
+; CHECK-NEXT:    [[LSHR:%.*]] = add nuw i32 [[TMP1]], [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[LSHR]]
+;
+  %shl = shl nuw i32 %x, %c
+  %add = add nuw i32 %shl, %y
+  %lshr = lshr i32 %add, %c
+  ret i32 %lshr
+}
+
+define i32 @shl_add_lshr_comm(i32 %x, i32 %c, i32 %y) {
+; CHECK-LABEL: @shl_add_lshr_comm(
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[Y:%.*]], [[C:%.*]]
+; CHECK-NEXT:    [[LSHR:%.*]] = add nuw i32 [[TMP1]], [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[LSHR]]
+;
+  %shl = shl nuw i32 %x, %c
+  %add = add nuw i32 %y, %shl
+  %lshr = lshr i32 %add, %c
+  ret i32 %lshr
+}
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nikic wrote:

This test does not actually test  the commuted case, because it gets canonicalized, see https://llvm.godbolt.org/z/93sGo78W8.

Read https://llvm.org/docs/InstCombineContributorGuide.html#add-commuted-tests to understand how to resolve this issue.

Also, your pre-commit tests don't show this because your second commit modifies the test inputs as well. The split is to add all tests in the first commit and only change CHECK lines in the second commit. Do not modify test inputs in the second commit.

https://github.com/llvm/llvm-project/pull/88193