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Stephan Plöger via llvm-dev

2016-Dec-19 16:41 UTC

[llvm-dev] opt -instcombine interesting behavior

Hi all,

I've been playing around with the llvm optimizer and stumbled upon an
interesting behavior with respect
to the "instcombine" option. I tried to optimize the following small
program (see also attachment small.ll)
(only with "instcombine"):

clang -v
clang version 4.0.0 (trunk 288238)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.4.0
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6.0.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64


llvm-as small.ll -o small.bc
opt -instcombine small.bc -o smallOpt.bc


define i32 @main() {
  %reg = alloca i32, i32 0
  store i32 1, i32* %reg
  %1 = getelementptr i32, i32* %reg, i32 0
  %2 = bitcast i32* %1 to [4 x i8]*
  %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
  store i8 1, i8* %3
  %4 = load i32, i32* %reg
  ret i32 %4
}

The program in pseudo code would/should simplified look something like this:

int main(){
    reg = 1;
    [reg+1] = 1; //Second byte of reg is set to 1
    return reg;
}

I assumed it would be optimized to - again pseudo code -:

int main(){
    return 257;
}

Instead, the program is optimized to(see also attachment
smallOpt.ll/smallOpt.bc):

define i32 @main() {
  ret i32 1
}

So i verified my assumption by printing '%4' of the original program
(smallPrint.ll/smallPrint.bc) and the output was 257.
I also optimized smallPrint.ll/smallPrint.bc , again only with
instcombine, but the output is changed to 1 (smallPrintOpt.bc).

I also tested it with clang 3.8.0.

What am i missing?


Best regards,
Stephan
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@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00"



declare i32 @printf(i8*, ...) #1


define i32 @main() {
  %reg = alloca i32, i32 0
  store i32 1, i32* %reg
  %1 = getelementptr i32, i32* %reg, i32 0
  %2 = bitcast i32* %1 to [4 x i8]*
  %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
  store i8 1, i8* %3
  %4 = load i32, i32* %reg
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i32 0, i32 0), i32 %4)
  ret i32 %4
}
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; ModuleID = 'smallOpt.bc'
source_filename = "small.ll"

define i32 @main() {
  ret i32 1
}
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define i32 @main() {
  %reg = alloca i32, i32 0
  store i32 1, i32* %reg
  %1 = getelementptr i32, i32* %reg, i32 0
  %2 = bitcast i32* %1 to [4 x i8]*
  %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
  store i8 1, i8* %3
  %4 = load i32, i32* %reg
  ret i32 %4
}

Tim Northover via llvm-dev

2016-Dec-19 17:13 UTC

head link

[llvm-dev] opt -instcombine interesting behavior

Hi Stephan,

On 19 December 2016 at 08:41, Stephan Plöger via llvm-dev
<llvm-dev at lists.llvm.org> wrote:>   %reg = alloca i32, i32 0
This line is allocating 0 bytes rather than allocating an i32 and
setting the memory to 0, so the rest of the function is really dodgy.
I'm not sure exactly why InstCombine decides on 1, but really anything
could have happened.

Cheers.

Tim.

Nema, Ashutosh via llvm-dev

2016-Dec-19 17:51 UTC

head link

[llvm-dev] opt -instcombine interesting behavior

>   %reg = alloca i32, i32 0Allocating 0 bytes is legal but it's an undefined behavior, please refer:
http://llvm.org/docs/LangRef.html#id184

You need to correct allocation to get the desired output,
i.e.:>   %reg = alloca i32, i32 1
Regards,
Ashutosh
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Stephan Plöger via llvm-dev
> Sent: Monday, December 19, 2016 10:11 PM
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] opt -instcombine interesting behavior
> 
> Hi all,
> 
> I've been playing around with the llvm optimizer and stumbled upon an
> interesting behavior with respect to the "instcombine" option. I
tried to
> optimize the following small program (see also attachment small.ll) (only
> with "instcombine"):
> 
> clang -v
> clang version 4.0.0 (trunk 288238)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/local/bin
> Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5 Found
> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.4.0 Found
> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6 Found candidate
> GCC installation: /usr/lib/gcc/i686-linux-gnu/6.0.0 Found candidate GCC
> installation: /usr/lib/gcc/x86_64-linux-gnu/5 Found candidate GCC
> installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6 Found
> candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
> Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
> Candidate multilib: .;@m64
> Candidate multilib: 32;@m32
> Candidate multilib: x32;@mx32
> Selected multilib: .;@m64
> 
> 
> llvm-as small.ll -o small.bc
> opt -instcombine small.bc -o smallOpt.bc
> 
> 
> define i32 @main() {
>   %reg = alloca i32, i32 0
>   store i32 1, i32* %reg
>   %1 = getelementptr i32, i32* %reg, i32 0
>   %2 = bitcast i32* %1 to [4 x i8]*
>   %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
>   store i8 1, i8* %3
>   %4 = load i32, i32* %reg
>   ret i32 %4
> }
> 
> The program in pseudo code would/should simplified look something like
> this:
> 
> int main(){
>     reg = 1;
>     [reg+1] = 1; //Second byte of reg is set to 1
>     return reg;
> }
> 
> I assumed it would be optimized to - again pseudo code -:
> 
> int main(){
>     return 257;
> }
> 
> Instead, the program is optimized to(see also attachment
> smallOpt.ll/smallOpt.bc):
> 
> define i32 @main() {
>   ret i32 1
> }
> 
> So i verified my assumption by printing '%4' of the original
program
> (smallPrint.ll/smallPrint.bc) and the output was 257.
> I also optimized smallPrint.ll/smallPrint.bc , again only with instcombine,
but
> the output is changed to 1 (smallPrintOpt.bc).
> 
> I also tested it with clang 3.8.0.
> 
> What am i missing?
> 
> 
> Best regards,
> Stephan

Mehdi Amini via llvm-dev

2016-Dec-19 18:01 UTC

head link

[llvm-dev] opt -instcombine interesting behavior

> On Dec 19, 2016, at 9:51 AM, Nema, Ashutosh via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
>>  %reg = alloca i32, i32 0
> Allocating 0 bytes is legal but it's an undefined behavior, please
refer:
> http://llvm.org/docs/LangRef.html#id184
<http://llvm.org/docs/LangRef.html#id184>
Nit: I believe that when LangRef says “the result is undefined”, it does not
mean “undefined behavior”.
See http://llvm.org/docs/LangRef.html#undefined-values
<http://llvm.org/docs/LangRef.html#undefined-values> for more details.
> 
> You need to correct allocation to get the desired output, i.e.:
>>  %reg = alloca i32, i32 1
The “, i32 1” is optional though (default is 1).

— 
Mehdi


> 
> Regards,
> Ashutosh
> 
>> -----Original Message-----
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>> Stephan Plöger via llvm-dev
>> Sent: Monday, December 19, 2016 10:11 PM
>> To: llvm-dev at lists.llvm.org
>> Subject: [llvm-dev] opt -instcombine interesting behavior
>> 
>> Hi all,
>> 
>> I've been playing around with the llvm optimizer and stumbled upon
an
>> interesting behavior with respect to the "instcombine"
option. I tried to
>> optimize the following small program (see also attachment small.ll)
(only
>> with "instcombine"):
>> 
>> clang -v
>> clang version 4.0.0 (trunk 288238)
>> Target: x86_64-unknown-linux-gnu
>> Thread model: posix
>> InstalledDir: /usr/local/bin
>> Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5 Found
>> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.4.0 Found
>> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6 Found
candidate
>> GCC installation: /usr/lib/gcc/i686-linux-gnu/6.0.0 Found candidate GCC
>> installation: /usr/lib/gcc/x86_64-linux-gnu/5 Found candidate GCC
>> installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
>> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6 Found
>> candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
>> Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
>> Candidate multilib: .;@m64
>> Candidate multilib: 32;@m32
>> Candidate multilib: x32;@mx32
>> Selected multilib: .;@m64
>> 
>> 
>> llvm-as small.ll -o small.bc
>> opt -instcombine small.bc -o smallOpt.bc
>> 
>> 
>> define i32 @main() {
>>  %reg = alloca i32, i32 0
>>  store i32 1, i32* %reg
>>  %1 = getelementptr i32, i32* %reg, i32 0
>>  %2 = bitcast i32* %1 to [4 x i8]*
>>  %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
>>  store i8 1, i8* %3
>>  %4 = load i32, i32* %reg
>>  ret i32 %4
>> }
>> 
>> The program in pseudo code would/should simplified look something like
>> this:
>> 
>> int main(){
>>    reg = 1;
>>    [reg+1] = 1; //Second byte of reg is set to 1
>>    return reg;
>> }
>> 
>> I assumed it would be optimized to - again pseudo code -:
>> 
>> int main(){
>>    return 257;
>> }
>> 
>> Instead, the program is optimized to(see also attachment
>> smallOpt.ll/smallOpt.bc):
>> 
>> define i32 @main() {
>>  ret i32 1
>> }
>> 
>> So i verified my assumption by printing '%4' of the original
program
>> (smallPrint.ll/smallPrint.bc) and the output was 257.
>> I also optimized smallPrint.ll/smallPrint.bc , again only with
instcombine, but
>> the output is changed to 1 (smallPrintOpt.bc).
>> 
>> I also tested it with clang 3.8.0.
>> 
>> What am i missing?
>> 
>> 
>> Best regards,
>> Stephan
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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