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

[Philip Reames]

I came in to an email this morning that said basically the same thing 
for the reduced example we were looking at.  However, the original IR it 
came from (before hand reduction) had the data layout set correctly, so 
there's probably still *something* going on.  It's just not what I 
thought at first.  :)

Philip


On 09/10/2014 02:26 AM, Roel Jordans wrote:
> Looking at the -debug output of opt shows that SROA was skipped due to 
> missing target data.
>
> Adding something like:
>
> target datalayout = "e-p:32:32:32-i32:32:32"
>
> to the top seems sufficient to fix the issue at -O3.
>
> By defining the size and storage requirements for i32 SROA is capable 
> of rewriting the array load into a constant scalar load which can then 
> be further optimized.
>
> Cheers,
>  Roel
>
> On 09/09/14 18:30, 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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