Peng Cheng
2014-Sep-09 16:30 UTC
[LLVMdev] failed folding with constant array with opt -O3
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140909/d90c01d7/attachment.html>
Philip Reames
2014-Sep-10 03:05 UTC
[LLVMdev] failed folding with constant array with opt -O3
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 > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140909/cc9f076f/attachment.html>
Roel Jordans
2014-Sep-10 09:26 UTC
[LLVMdev] failed folding with constant array with opt -O3
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 > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Philip Reames
2014-Sep-10 16:50 UTC
[LLVMdev] failed folding with constant array with opt -O3
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 >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
Seemingly Similar Threads
- [LLVMdev] failed folding with constant array with opt -O3
- Potential issue with noalias @malloc and @realloc
- [LLVMdev] Why is the loop vectorizer not working on my function?
- [LLVMdev] Why is the loop vectorizer not working on my function?
- [LLVMdev] Why is the loop vectorizer not working on my function?