[cfe-dev] [llvm-dev] Rewriting calls to varargs functions

[Dean Michael Berris via cfe-dev]

I can imagine it being useful to make suggestions as a clang-tidy check, rather than an “optimisation”. In particular, you might want to suggest to callers of printf that they might as well fold in constants into the string, and maybe using puts instead.

> On 23 May 2018, at 01:42, Dávid Bolvanský via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Thanks.
> 
> Yes, to substitute only some of the arguments. Formatting used by printf depends on the locale but only for double, float types I think - yes, I would not place double/float constants into the format string.
> 
> Why? To reduce number of constants (some of them could be merged into the format string) and number of args when calling printf/fprintf/sprintf, etc..
> 
> 2018-05-22 16:22 GMT+02:00 Hal Finkel <hfinkel at anl.gov>:
> 
> On 05/22/2018 04:32 AM, Dávid Bolvanský via llvm-dev wrote:
>> Hello,
>> 
>> A new patch:
>> https://reviews.llvm.org/D47159
>> 
>> proposes transformations like:
>> printf("Hello, %s %d", "world", 123) - > printf("Hello world 123")
> 
> To clarify, the real question here comes up when you can only substitute some of the arguments? If you can substitute all of the arguments, then you can turn this into a call to puts.
> 
> In any case , why do you want to do this? Also, doesn't the formatting used by printf depend on the process's current locale?
> 
>  -Hal
> 
>> 
>> As Eli noted:
>> 
>> "I'm not sure we can rewrite calls to varargs functions safely in general given the current state of the C ABI rules in LLVM.
>> 
>> Sometimes clang does weird things to conform with the ABI rules, because the LLVM type system isn't the same as the C system. For most functions, it's pretty easy to tell it happened: if the IR signature of the function doesn't match the expected signature, something weird happened, so we can just bail out. But varargs functions don't specify a complete signature, so we can't tell if the clang ABI code was forced to do something weird, like split an argument into multiple values, or insert a padding value. For example, for the target mips64-unknown-linux-gnu, a call like printf("asdf%Lf", 1.0L); gets lowered to the following:
>> 
>> %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([5 x i8], [5 x i8]* @.str, i32 0, i32 0), i64 undef, fp128 0xL00000000000000003FFF000000000000) #2"
>> 
>> 
>> I would to hear more suggestions whether it is safe or not. Seems like for mips Clang produces some weird IR, but e.g. x86 IR seems ok.
>> 
>> Any folks from Clang/LLVM to bring more information about "varargs vs ABI vs LLVM vs Clang"? 
>> And whether we can rewrite calls to varargs functions safely under some conditions..
>> 
>> Thanks
>> 
>> 
>> 
>> 
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> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 
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-- Dean