[PATCH] D102116: [LoopIdiom] 'logical right-shift until zero' ('count active bits') "on steroids" idiom recognition.

[Han Zhu via Phabricator via llvm-commits]

zhuhan0 added inline comments.


================
Comment at: llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll:9-65
 ; Most basic pattern; Note that iff the shift amount is offset, said offsetting
 ; must not cause an overflow, but `add nsw` is fine.
 define i8 @p0(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @p0(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
----------------
I could be wrong but would this mis-compile if %nbits results in unsigned overflow? For example,
```
%val = 0x10000000
%start = 1
%extraoffset.= 255
```
Loop trip count is 8 before transformation but 1 after.


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  https://reviews.llvm.org/D102116/new/

https://reviews.llvm.org/D102116