[LLVMdev] failed folding with constant array with opt -O3

[Philip Reames]

One of my coworkers ran across a very similar case as well.  He's still 
reducing his test case to identify the problematic area, but based on 
the initial results, it looks like we have an overly conservative 
aliasing result being returned for two locations in disjoint elements of 
an array.  Your example is slightly different, but an imprecise alias 
query (i.e. not recognizing that the load is completely covered by the 
store) might explain it as well.

If you could file a bug with your example, that would be helpful. If you 
can further reduce it that would be nice, but you've already gotten it 
down small enough to be useful.  Thanks.

Philip

On 09/09/2014 09:30 AM, Peng Cheng wrote:
> I have the following simplified llvm ir, which basically returns value 
> based on the first value of a constant array.
>
> ----
> ; ModuleID = 'simple_ir3.txt'
>
> @f.b = constant [1 x i32] [i32 1], align 4          ; constant array 
> with value 1 at the first element
>
> define void @f(i32* nocapture %l0) {
> entry:
>   %fc_ = alloca [1 x i32]
>   %f.b.v = load [1 x i32]* @f.b
>   store [1 x i32] %f.b.v, [1 x i32]* %fc_
>   %0 = getelementptr [1 x i32]* %fc_, i64 0, i64 0  ; load the first 
> element of the constant array, which is actually 1
>   %1 = load i32* %0
>   %tobool = icmp ne i32 %1, 0             ; check the first element to 
> see if it is 1, which is actually always true since the first element 
> of constant array is 1
>   br i1 %tobool, label %2, label %4
>
> ; <label>:2               ; true branch
>   store i32 1, i32* %l0;
>   %3 = load i32* %l0;
>   br label %4
>
> ; <label>:4
>   %storemerge = phi i32 [ %3, %2 ], [ 0, %entry ]
>   store i32 %storemerge, i32* %l0
>   ret void
> }
> ---
>
> I ran opt -O3 simple_ir.txt -S, and got:
>
> ---
> ; ModuleID = 'simple_ir3.txt'
>
> @f.b = constant [1 x i32] [i32 1], align 4
>
> ; Function Attrs: nounwind
> define void @f(i32* nocapture %l0) #0 {
> entry:
>   %fc_ = alloca [1 x i32]
>   store [1 x i32] [i32 1], [1 x i32]* %fc_
>   %0 = getelementptr [1 x i32]* %fc_, i64 0, i64 0
>   %1 = load i32* %0
>   %tobool = icmp eq i32 %1, 0
>   br i1 %tobool, label %3, label %2
>
> ; <label>:2                                       ; preds = %entry
>   store i32 1, i32* %l0
>   br label %3
>
> ; <label>:3                                       ; preds = %entry, %2
>   %storemerge = phi i32 [ 1, %2 ], [ 0, %entry ]
>   store i32 %storemerge, i32* %l0
>   ret void
> }
>
> attributes #0 = { nounwind }
> ---
>
> I would expect that the constant folding, or some other 
> transformations, would be able to fold the constant to get the 
> following ir:
>
> ---
> define void @f(i32* nocapture %l0) #0 {
>   store i32 1, i32* %l0
>   ret void
> }
> ---
>
> How could I get the expected optimized ir?  update the original ir, or 
> use different set of transformations?
>
> Any suggestions or comments?
>
>
> Thanks,
> -Peng
>
>
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