[LLVMdev] failed folding with constant array with opt -O3
Peng Cheng
gm4cheng at gmail.com
Wed Sep 10 18:55:38 PDT 2014
Thanks for the inputs! Adding basicaa works. And extra transforms got the
ideal optimized code.
I originally thought GVN has AG dependency on alias analysis, so it should
get the expected elimination. While digging into the code, it turned out
that NoAA is provided to GVN by default. Therefore, to get meaningful
results from GVN, basicaa is needed.
In summary, to get the original ir to the optimized code, there are two
sets of transformations. One is through sroa, which needs the target
layout; and the other is using gvn, which needs a nontrivia aa.
Any thoughts on which one is cheaper in computational cost to eliminate the
load? sroa seems the one to me.
Best,
-Peng
On Wed, Sep 10, 2014 at 3:58 PM, Roel Jordans <r.jordans at tue.nl> wrote:
> Adding some form of alias analysis like -basicaa to the list should solve
> the clobbering issue.
>
> You're probably also interested in some the following to get your code
> completely optimized again: -constprop -simplifycfg -dse
>
> Cheers,
> Roel
>
>
> On 10/09/14 21:00, Peng Cheng wrote:
>
>> Thanks for your help! Providing data layout works for the example I have.
>>
>> I also feel there is still something else going on during the
>> investigation.
>>
>> I have another simple ir, which basically does the same thing as the
>> original example except initialize the array element by element instead
>> of by an constant array.
>>
>> ---
>> define void @f(i32* nocapture %l0) {
>> entry:
>> %fc_ = alloca [1 x i32]
>> %0 = getelementptr inbounds [1 x i32]* %fc_, i32 0, i32 0
>> store i32 1, i32* %0, align 4
>> %1 = getelementptr [1 x i32]* %fc_, i64 0, i64 0
>> %2 = load i32* %1
>> %tobool = icmp eq i32 %2, 0
>> br i1 %tobool, label %4, label %3
>>
>> ; <label>:3 ; preds = %entry
>> store i32 1, i32* %l0
>> br label %4
>>
>> ; <label>:4 ; preds = %entry, %3
>> %storemerge = phi i32 [ 1, %3 ], [ 0, %entry ]
>> store i32 %storemerge, i32* %l0
>> ret void
>> }
>> ---
>>
>> For this ir, without target data layout,
>> opt -O2 got the expected optmization:
>>
>> ---
>> ; ModuleID = 't1.txt'
>>
>> ; Function Attrs: nounwind
>> define void @f(i32* nocapture %l0) #0 {
>> store i32 1, i32* %l0
>> ret void
>> }
>>
>> attributes #0 = { nounwind }
>> ---
>>
>> By checking the ir after each transformation, I see that gvn removes the
>> load expression and the transformations before do nothing.
>>
>> So, I ran opt -gvn on the ir, but did not get load expression eliminated.
>>
>> Checking into the gvn code. Looks like the memory dependency
>> computation got different results for the load expression with -O2 or
>> -gvn. With O2, the load is not clobbered, but with gvn alone, it says
>> clobbered.
>>
>> Does that sound expected?
>>
>>
>>
>>
>>
>>
>> On Wed, Sep 10, 2014 at 12:50 PM, Philip Reames
>> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>>
>> 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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