[llvm-dev] Rewriting calls to varargs functions

Tue May 22 17:11:09 PDT 2018

Converting to puts is usually not possible: puts appends a newline to its
output. The only really appropriate thing to convert to, that works in
general, is fwrite. But we can't convert to that because we can't form the
'stdout' parameter (stdout might be a macro rather than a global, or might
have a nontrivial mangling, so LLVM can't synthesize it). Also, converting
printf("Hello, %s", "world") to printf("Hello, world") is likely a
pessimization rather than an optimization for performance: printing a
string via %s just needs to write the string, whereas printing a format
string needs to scan for %s.

Having said all that, the opposite conversion (from printf("Hello, %s",
"world") to printf("%s", "Hello, world")) may be marginally worthwhile. And
there are some non-trivial tradeoffs here if you want to optimize for size.
(Eg, some format string refactorings may permit more string constant reuse.)

On 22 May 2018 at 10:26, Hubert Tong via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> On Tue, May 22, 2018 at 12:59 PM, Dávid Bolvanský via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> It could save useless parsing in s/f/printf during runtime.
>>
> A mix of calls to puts and calls to printf with format strings containing
> just a conversion specifier can help towards such a goal without mutating
> constants beyond the format string.
>
>
>>
>> E.g. for heavy "fprint"ing code like fprintf(f, "%s: %s", TAG, msg); I
>> think it could be quite useful.
>> After this transformation we would get fprintf(f, "ABC: %s", msg);  -->
>> We could save one push/mov instruction + less parsing in printf every time
>> we call it. We would just replace string constant "%s: %s" with "ABC: %s"
>> and possibly orphaned "ABC" constant could be removed completely.
>>
>>
>>
>> 2018-05-22 18:36 GMT+02:00 Hal Finkel <hfinkel at anl.gov>:
>>
>>>
>>> On 05/22/2018 10:42 AM, Dávid Bolvanský 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.
>>>
>>>
>>> Okay. I think it's true that integers will be the same regardless of
>>> locale (so long as the ' flag is not used, as that brings in a dependence
>>> on LC_NUMERIC).
>>>
>>>
>>> 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..
>>>
>>>
>>> Sure, but it seems to me unlikely that this will affect performance. Is
>>> it a code-size optimization (this actually isn't obvious to me because the
>>> string representation might be longer than the binary form of the constant
>>> plus the extra instructions)?
>>>
>>>  -Hal
>>>
>>>
>>>
>>> 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
>>>>
>>>>
>>>
>>> --
>>> Hal Finkel
>>> Lead, Compiler Technology and Programming Languages
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>>>
>>>
>>
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