llvm dev - Oct 2012 - [LLVMdev] [llvm-commits] [llvm] r166875 - in /llvm/trunk: lib/Transforms/Scalar/LoopIdiomRecognize.cpp test/Transforms/LoopIdiom/basic.ll

On 27.10.2012, at 18:24, Benjamin Kramer <benny.kra at gmail.com> wrote:
> 
> On 27.10.2012, at 18:15, Sean Silva <silvas at purdue.edu> wrote:
> 
>> How does this affect freestanding implementations?
> 
> This transform is disabled with -fno-builtin or -ffreestanding.
Thinking a bit more about this, it looks like the way -ffreestanding is
implemented in clang is too conservative. Currently -ffreestanding disables
optimizing all builtin functions, just like -fno-builtin. However, the GCC
manual says:
> GCC requires the freestanding environment provide memcpy,memmove, memset
and memcmp.
Should we enable those functions selectively in a freestanding environment? LLVM
will start optimizing calls to them, and turn loops into one of those functions
if possible. I'm not sure if that behavior is acceptable with
-ffreestanding.

- Ben

If GCC requires it, then pragmatically it's probably safe to do. Is
there any -ffreestanding code that LLVM compiles that wasn't already
being compiled with a GCC toolchain? My guess is that there isn't.

A relevant related question is what freestanding code does LLVM
currently compile in production? FreeBSD kernel and Darwin kernel come
to mind (and Linux is coming along). We also have a couple
microcontroller backends, but given the lack of maintenance (from a
quick look at git log), I'm not sure how much they are being used.
Maybe some embedded ARM and MIPS systems. AFAIK, all of these come
from a background of being built with a GCC toolchain.

-- Sean Silva

On Sat, Oct 27, 2012 at 4:32 PM, Benjamin Kramer <benny.kra at gmail.com>
wrote:
>
> On 27.10.2012, at 18:24, Benjamin Kramer <benny.kra at gmail.com>
wrote:
>
>>
>> On 27.10.2012, at 18:15, Sean Silva <silvas at purdue.edu> wrote:
>>
>>> How does this affect freestanding implementations?
>>
>> This transform is disabled with -fno-builtin or -ffreestanding.
>
> Thinking a bit more about this, it looks like the way -ffreestanding is
implemented in clang is too conservative. Currently -ffreestanding disables
optimizing all builtin functions, just like -fno-builtin. However, the GCC
manual says:
>
>> GCC requires the freestanding environment provide memcpy,memmove,
memset and memcmp.
>
> Should we enable those functions selectively in a freestanding environment?
LLVM will start optimizing calls to them, and turn loops into one of those
functions if possible. I'm not sure if that behavior is acceptable with
-ffreestanding.
>
> - Ben

I think it would be possible according to the gcc spec, but still a bad idea. 
We regularly get bug reports from people using freestanding and getting grumpy
about getting calls to memcpy for struct copies...

I think that if someone is asking for freestanding we should just forget about
any performance win for stuff like this and be as least surprising as possible.

-Chris

On Oct 27, 2012, at 10:32 AM, Benjamin Kramer <benny.kra at gmail.com>
wrote:
> 
> On 27.10.2012, at 18:24, Benjamin Kramer <benny.kra at gmail.com>
wrote:
> 
>> 
>> On 27.10.2012, at 18:15, Sean Silva <silvas at purdue.edu> wrote:
>> 
>>> How does this affect freestanding implementations?
>> 
>> This transform is disabled with -fno-builtin or -ffreestanding.
> 
> Thinking a bit more about this, it looks like the way -ffreestanding is
implemented in clang is too conservative. Currently -ffreestanding disables
optimizing all builtin functions, just like -fno-builtin. However, the GCC
manual says:
> 
>> GCC requires the freestanding environment provide memcpy,memmove,
memset and memcmp.
> 
> Should we enable those functions selectively in a freestanding environment?
LLVM will start optimizing calls to them, and turn loops into one of those
functions if possible. I'm not sure if that behavior is acceptable with
-ffreestanding.
> 
> - Ben
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