[LLVMdev] [MC] Removing relaxation control

[Daniel Dunbar]

Yeah, I would rather not turn it off.

Note that in the context of *just* the assembler time, it has a pretty
large impact:
--
ddunbar at ozzy:MC-403.gcc$ runN 10
/Users/ddunbar/llvm.obj.64/Release/bin/clang -m32 -c -O0 i386.normal.s
name	  avg  	  min  	  med  	  max  	   SD  	 total
user	 0.1753	 0.1729	 0.1752	 0.1768	 0.0010	 1.7533
 sys	 0.0221	 0.0247	 0.0222	 0.0225	 0.0011	 0.2210
wall	 0.2580	 0.3186	 0.2560	 0.2593	 0.0220	 2.5800
ddunbar at ozzy:MC-403.gcc$ runN 10
/Users/ddunbar/llvm.obj.64/Release/bin/clang -m32 -c -O0 i386.normal.s
-mno-relax-all
name	  avg  	  min  	  med  	  max  	   SD  	 total
user	 0.1883	 0.1853	 0.1878	 0.1927	 0.0023	 1.8831
 sys	 0.0283	 0.0287	 0.0266	 0.0262	 0.0016	 0.2828
wall	 0.3247	 0.2869	 0.3326	 0.3295	 0.0463	 3.2474
ddunbar at ozzy:MC-403.gcc$
--
That's 10% faster, which is a big deal.

Overall, of course, the assembler time isn't that much of the total
compile time, so the overall impact isn't that large, but I like
following the model of trying to keep individual components as fast as
possible.

Note the impact that -mrelax-all has on the assembler stats:
--
ddunbar at ozzy:MC-403.gcc$ clang -m32 -c -w -DSPEC_CPU -DSPEC_CPU_MACOSX
-I. -O0 i386.c -mllvm -stats 2>&1 | grep assembler
     1 assembler     - Number of assembler layout and relaxation steps
  1182 assembler     - Number of emitted assembler fragments
279600 assembler     - Number of emitted object file bytes
  9090 assembler     - Number of evaluated fixups
  1201 assembler     - Number of fragment layouts

ddunbar at ozzy:MC-403.gcc$ clang -m32 -c -w -DSPEC_CPU -DSPEC_CPU_MACOSX
-I. -O0 i386.c -mllvm -stats -mno-relax-all 2>&1 | grep assembler
     2 assembler     - Number of assembler layout and relaxation steps
  9648 assembler     - Number of emitted assembler fragments
267264 assembler     - Number of emitted object file bytes
 23872 assembler     - Number of evaluated fixups
 24604 assembler     - Number of fragment layouts
  1061 assembler     - Number of relaxed instructions
ddunbar at ozzy:MC-403.gcc$
--
We get about 8x the number of fragments and 20x the number of fragment layouts.

 - Daniel

On Sat, Feb 26, 2011 at 11:51 AM, Chris Lattner <clattner at apple.com> wrote:
>
> On Feb 25, 2011, at 11:38 AM, Rafael Avila de Espindola wrote:
>
>>>> Can someone else try to reproduce this?
>>
>> I tried gcc.c from
>> http://people.csail.mit.edu/smcc/projects/single-file-programs/ and the
>> difference is a bit more noticeable:
>>
>> -O0 -mno-relax-all
>>
>> real  0m13.182s
>> user  0m12.690s
>> sys   0m0.450s
>>
>> -O0
>>
>> gcc.o is 10932968 bytes.
>>
>> real  0m12.969s
>> user  0m12.520s
>> sys   0m0.410s
>>
>> gcc.o is 11410552 bytes
>>
>> IMHO it would still be reasonable to switch to no-relax-all, but I am
>> probably not the best one to make the call.
>
> That looks like a 1.5% speedup in realtime and 10% speedup in system time (though I'm not sure I believe that).  I think it should stay on for -O0 for C files.  Turning it off at -O0 for .s files makes perfect sense to me though.
>
> -Chris
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