llvm dev - Dec 2009 - [LLVMdev] detailed comparison of generated code size for LLVM and other compilers

2009/12/14 Chris Lattner <clattner at apple.com>:
> I'd recommend targeting (with both -march and -mtune) a simple and
> commonly available CPU type like "core2" or "pentium4".
 ICC should
> have both of these and gcc/llvm definitely do.
While I would say that, to be fair, the comparison should be made with
the same options (-O3 only or something of the sort), ICC is specific
for Intel and GCC is highly tuned to, which is not the case of LLVM.
Still, if the target is specified, I'd assume we should enable all
tested and proven (AFAP) optimizations to that particular platform by
default.

So, my question is: are those optimizations turned off by default
because they're experimental?

--renato

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On Mon, Dec 14, 2009 at 3:29 PM, Renato Golin <rengolin at systemcall.org>
wrote:
> 2009/12/14 Chris Lattner <clattner at apple.com>:
>> I'd recommend targeting (with both -march and -mtune) a simple and
>> commonly available CPU type like "core2" or
"pentium4".  ICC should
>> have both of these and gcc/llvm definitely do.
>
> While I would say that, to be fair, the comparison should be made with
> the same options (-O3 only or something of the sort), ICC is specific
> for Intel and GCC is highly tuned to, which is not the case of LLVM.
> Still, if the target is specified, I'd assume we should enable all
> tested and proven (AFAP) optimizations to that particular platform by
> default.
>
> So, my question is: are those optimizations turned off by default
> because they're experimental?
The issue here is more arbitrary differences due to different default
code generation choices; for example, clang defaults to generating
SSE2 code, while llvm-gcc defaults to using x87 FP.

-Eli

> The issue here is more arbitrary differences due to different default
> code generation choices; for example, clang defaults to generating
> SSE2 code, while llvm-gcc defaults to using x87 FP.
Aha, this explains some apparently bizarre results such as the second one 
(018427, d) on this page:

http://embed.cs.utah.edu/embarrassing/dec_09/harvest/llvm-gcc-head_clang-head/

I had been wondering about this one.

John