llvm dev - May 2018 - Constants propagation into printf format string

Hello,

I was thinking about a new possible simplification in InstCombine which
would transform code like this:

const char * NAME = "Prog";

void printer(char **arr, int len) {
  for (int i = 0; i < len; ++i) {
    printf("%s: %s", NAME, arr[i]);
  }
}

into

void printer(char **arr, int len) {
  for (int i = 0; i < len; ++i) {
    printf("Prog: %s", arr[i]);
  }
}

This transformation would take constant strings/integers/chars and put them
to the format string.

In the example I used "printf" but it could be applied also for
fprintf and sprintf.

Do you consider this as good to be implemented? Or not worth to do at all?

I implemented it some time ago personally just for fun to try it. I would
reimplement it and send a patch to LLVM if any interest.

Ideas? Complains? Suggestions?

Thanks
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On Thu, May 17, 2018 at 3:53 PM, Dávid Bolvanský via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hello,
>
> I was thinking about a new possible simplification in InstCombine which
> would transform code like this:
>
> const char * NAME = "Prog";
>
> void printer(char **arr, int len) {
>   for (int i = 0; i < len; ++i) {
>     printf("%s: %s", NAME, arr[i]);
>   }
> }
>
> into
>
> void printer(char **arr, int len) {
>   for (int i = 0; i < len; ++i) {
>     printf("Prog: %s", arr[i]);
>   }
> }
>
> This transformation would take constant strings/integers/chars and put them
> to the format string.
>
This is not a peephole optimization, therefore it doesn't belong to
instcombine.
If you want to try to implement something like this, I'd recommend
taking a look at ConstantFolding or SCCP (and try to understand why
the value doesn't get propagated).

Thanks,

--
Davide

Thanks, I will look at it.

2018-05-18 1:04 GMT+02:00 Davide Italiano <davide at freebsd.org>:
> On Thu, May 17, 2018 at 3:53 PM, Dávid Bolvanský via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hello,
> >
> > I was thinking about a new possible simplification in InstCombine
which
> > would transform code like this:
> >
> > const char * NAME = "Prog";
> >
> > void printer(char **arr, int len) {
> >   for (int i = 0; i < len; ++i) {
> >     printf("%s: %s", NAME, arr[i]);
> >   }
> > }
> >
> > into
> >
> > void printer(char **arr, int len) {
> >   for (int i = 0; i < len; ++i) {
> >     printf("Prog: %s", arr[i]);
> >   }
> > }
> >
> > This transformation would take constant strings/integers/chars and put
> them
> > to the format string.
> >
>
> This is not a peephole optimization, therefore it doesn't belong to
> instcombine.
> If you want to try to implement something like this, I'd recommend
> taking a look at ConstantFolding or SCCP (and try to understand why
> the value doesn't get propagated).
>
> Thanks,
>
> --
> Davide
>
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On 5/17/2018 4:04 PM, Davide Italiano via llvm-dev
wrote:
> On Thu, May 17, 2018 at 3:53 PM, Dávid Bolvanský via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hello,
>>
>> I was thinking about a new possible simplification in InstCombine which
>> would transform code like this:
>>
>> const char * NAME = "Prog";
>>
>> void printer(char **arr, int len) {
>>    for (int i = 0; i < len; ++i) {
>>      printf("%s: %s", NAME, arr[i]);
>>    }
>> }
>>
>> into
>>
>> void printer(char **arr, int len) {
>>    for (int i = 0; i < len; ++i) {
>>      printf("Prog: %s", arr[i]);
>>    }
>> }
>>
>> This transformation would take constant strings/integers/chars and put
them
>> to the format string.
>>
> This is not a peephole optimization, therefore it doesn't belong to
instcombine.
> If you want to try to implement something like this, I'd recommend
> taking a look at ConstantFolding or SCCP (and try to understand why
> the value doesn't get propagated).
What?  Transforming `printf("FOO: %s", "BAR")` to
`printf("FOO: BAR")`
looks like a peephole to me.

-Eli

-- 
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