[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:

It doesn't work, because we use fine-grained tokens, and it requires a 'classic' token to work.

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