[PATCH] [InstCombine] Teach how to fold a select into a cttz/ctlz with the 'is_zero_undef' flag cleared.

[Andrea Di Biagio]

Hi hfinkel, majnemer, RKSimon,

Hi all,

This patch teaches the Instruction Combiner how to fold a cttz/ctlz followed by a icmp plus select into a single cttz/ctlz with flag 'is_zero_undef' cleared.

Example:

```
  %a = tail call i32 @llvm.cttz.i32(i32 %x, i1 true)
  %b = icmp ne i32 %x, 0
  %c = select i1 %b, i32 %a, i32 32
```

In this example, the condition value used by the select instruction compares value %x for equality against zero.
Value %x is also used by the call to the llvm intrinsic cttz. So, the select instruction would propagate the sizeof in bits of %x (i.e. 32) if %x is zero.

The entire `cttz+icmp+select` sequence can be safely folded into:

```
  %c = tail call i32 @llvm.cttz.i32(i32 %x, i1 false)
```

Added test InstCombine/select-cmp-cttz-ctlz.ll.

Please let me know if ok to submit.

Thanks,
Andrea

http://reviews.llvm.org/D6891

Files:
  lib/Transforms/InstCombine/InstCombineSelect.cpp
  test/Transforms/InstCombine/intrinsics.ll
  test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D6891.17922.patch
Type: text/x-patch
Size: 13721 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150109/c1d694ae/attachment.bin>