[llvm] [InstCombine] fold icmp of select with invertible shl (PR #147182)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> > Miscompilation reproducer: https://alive2.llvm.org/ce/z/-TagCh
> 
> Is this patch causing this error? My compilation result is https://alive2.llvm.org/ce/z/enaJTE @dtcxzyw
> 
> ```
> define i1 @src(i64 %indvars.iv.i, i32 %conv161.i.i.i) {
> entry:
>   %cmp104.i.i.i = icmp slt i64 %indvars.iv.i, 1
>   br i1 %cmp104.i.i.i, label %lor.end157.i.i.i, label %lor.rhs106.i.i.i
> 
> lor.rhs106.i.i.i:
>   br label %lor.end157.i.i.i
> 
> lor.end157.i.i.i:
>   %conv159.i.i.i = zext i1 %cmp104.i.i.i to i16
>   %sub.i122.i.i.i = sub i16 0, %conv159.i.i.i
>   %conv161.i.i.i2 = zext i16 %sub.i122.i.i.i to i32
>   %shl.i.i.i.i = shl nuw i32 %conv161.i.i.i, 31
>   %cond.i124.i.i.i = select i1 %cmp104.i.i.i, i32 %conv161.i.i.i2, i32 %shl.i.i.i.i
>   %conv163.i.i.i = zext i32 %cond.i124.i.i.i to i64
>   %tobool166.not.i.i.i = icmp eq i64 %conv163.i.i.i, 0
>   ret i1 %tobool166.not.i.i.i
> }
> 
> define i1 @tgt(i64 %indvars.iv.i, i32 %conv161.i.i.i) {
> entry:
>   %cmp104.i.i.i = icmp sgt i64 %indvars.iv.i, 0
>   br i1 %cmp104.i.i.i, label %lor.rhs106.i.i.i, label %lor.end157.i.i.i
> 
> lor.rhs106.i.i.i:                                 ; preds = %entry
>   br label %lor.end157.i.i.i
> 
> lor.end157.i.i.i:                                 ; preds = %lor.rhs106.i.i.i, %entry
>   %tobool166.not.i.i.i1 = icmp eq i32 %conv161.i.i.i, 0
>   %tobool166.not.i.i.i = select i1 %cmp104.i.i.i, i1 %tobool166.not.i.i.i1, i1 false
>   ret i1 %tobool166.not.i.i.i
> }
> ```

Confirmed. I forgot to include the datalayout string in the reproducer. Sorry about this.
```
; bin/opt -passes=instcombine test.ll -S
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i1 @src(i64 %indvars.iv.i, i32 %conv161.i.i.i) {
entry:
  %cmp104.i.i.i = icmp slt i64 %indvars.iv.i, 1
  br i1 %cmp104.i.i.i, label %lor.end157.i.i.i, label %lor.rhs106.i.i.i

lor.rhs106.i.i.i:                                 ; preds = %entry
  br label %lor.end157.i.i.i

lor.end157.i.i.i:                                 ; preds = %lor.rhs106.i.i.i, %entry
  %conv159.i.i.i = zext i1 %cmp104.i.i.i to i16
  %sub.i122.i.i.i = sub i16 0, %conv159.i.i.i
  %conv161.i.i.i2 = zext i16 %sub.i122.i.i.i to i32
  %shl.i.i.i.i = shl nuw i32 %conv161.i.i.i, 31
  %cond.i124.i.i.i = select i1 %cmp104.i.i.i, i32 %conv161.i.i.i2, i32 %shl.i.i.i.i
  %conv163.i.i.i = zext i32 %cond.i124.i.i.i to i64
  %tobool166.not.i.i.i = icmp eq i64 %conv163.i.i.i, 0
  ret i1 %tobool166.not.i.i.i
}

```
With this patch:
```
define i1 @src(i64 %indvars.iv.i, i32 %conv161.i.i.i) {
entry:
  %cmp104.i.i.i = icmp slt i64 %indvars.iv.i, 1
  br i1 %cmp104.i.i.i, label %lor.end157.i.i.i, label %lor.rhs106.i.i.i

lor.rhs106.i.i.i:                                 ; preds = %entry
  %0 = icmp eq i32 %conv161.i.i.i, 0
  br label %lor.end157.i.i.i

lor.end157.i.i.i:                                 ; preds = %lor.rhs106.i.i.i, %entry
  %tobool166.not.i.i.i = phi i1 [ %0, %lor.rhs106.i.i.i ], [ true, %entry ]
  ret i1 %tobool166.not.i.i.i
}
```
Without this patch: https://alive2.llvm.org/ce/z/eL3WkX
```
define i1 @src(i64 %indvars.iv.i, i32 %conv161.i.i.i) {
entry:
  %cmp104.i.i.i = icmp slt i64 %indvars.iv.i, 1
  br i1 %cmp104.i.i.i, label %lor.end157.i.i.i, label %lor.rhs106.i.i.i

lor.rhs106.i.i.i:                                 ; preds = %entry
  br label %lor.end157.i.i.i

lor.end157.i.i.i:                                 ; preds = %lor.rhs106.i.i.i, %entry
  %conv161.i.i.i2 = phi i32 [ 0, %lor.rhs106.i.i.i ], [ 65535, %entry ]
  %shl.i.i.i.i = shl nuw i32 %conv161.i.i.i, 31
  %cond.i124.i.i.i = select i1 %cmp104.i.i.i, i32 %conv161.i.i.i2, i32 %shl.i.i.i.i
  %tobool166.not.i.i.i = icmp eq i32 %cond.i124.i.i.i, 0
  ret i1 %tobool166.not.i.i.i
}
```
If it is still not reproducible, use the base commit 77342761a9060159f09c0c6d5aa3da2e543a4877.


https://github.com/llvm/llvm-project/pull/147182