[llvm-dev] Rewriting calls to varargs functions

[Dávid Bolvanský via llvm-dev]

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
>
>
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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