[llvm-branch-commits] [llvm] release/18.x: [InstCombine] Drop nuw flag when CtlzOp is a sub nuw (#91776) (PR #91917)

[Tom Stellard via llvm-branch-commits]

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@@ -284,6 +284,42 @@ define <4 x i32> @bit_ceil_v4i32(<4 x i32> %x) {
   ret <4 x i32> %sel
 }
 
+define i32 @pr91691(i32 %0) {
+; CHECK-LABEL: @pr91691(
+; CHECK-NEXT:    [[TMP2:%.*]] = sub i32 -2, [[TMP0:%.*]]
+; CHECK-NEXT:    [[TMP3:%.*]] = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 [[TMP2]], i1 false)
+; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw i32 0, [[TMP3]]
+; CHECK-NEXT:    [[TMP5:%.*]] = and i32 [[TMP4]], 31
+; CHECK-NEXT:    [[TMP6:%.*]] = shl nuw i32 1, [[TMP5]]
+; CHECK-NEXT:    ret i32 [[TMP6]]
+;
+  %2 = sub nuw i32 -2, %0
+  %3 = tail call i32 @llvm.ctlz.i32(i32 %2, i1 false)
+  %4 = sub i32 32, %3
+  %5 = shl i32 1, %4
+  %6 = icmp ult i32 %0, -2
+  %7 = select i1 %6, i32 %5, i32 1
+  ret i32 %7
+}
+
+define i32 @pr91691_keep_nsw(i32 %0) {
+; CHECK-LABEL: @pr91691_keep_nsw(
+; CHECK-NEXT:    [[TMP2:%.*]] = sub nsw i32 -2, [[TMP0:%.*]]
+; CHECK-NEXT:    [[TMP3:%.*]] = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 [[TMP2]], i1 false)
+; CHECK-NEXT:    [[TMP4:%.*]] = sub nsw i32 0, [[TMP3]]
+; CHECK-NEXT:    [[TMP5:%.*]] = and i32 [[TMP4]], 31
+; CHECK-NEXT:    [[TMP6:%.*]] = shl nuw i32 1, [[TMP5]]
+; CHECK-NEXT:    ret i32 [[TMP6]]
+;
+  %2 = sub nsw i32 -2, %0
+  %3 = tail call i32 @llvm.ctlz.i32(i32 %2, i1 false)
+  %4 = sub i32 32, %3
+  %5 = shl i32 1, %4
+  %6 = icmp ult i32 %0, -2
+  %7 = select i1 %6, i32 %5, i32 1
+  ret i32 %7
+}
+
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tstellar wrote:

@nikic No, it's not. The problem is that you need two different types of permissions to be able to set that feature (write access to the branch and write access to the PR).  Because these both have different scopes (the branch is in the forked repository and the PR is in the main repository) it's not possible to use fine-grained tokens.  We'll have to switch back to the classic token to get it to work.  I'm planning to do this for the next release cycle.

https://github.com/llvm/llvm-project/pull/91917