[llvm-dev] Constants propagation into printf format string

[Friedman, Eli via llvm-dev]

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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