[llvm-dev] Rewriting calls to varargs functions

[Hal Finkel via llvm-dev]

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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