llvm dev - Feb 2011 - [LLVMdev] [MC] Removing relaxation control

Hi all,
clang -O0 currently differs from -O1 and higher in passing down
-mrelax-all. When compiling assembler files, this results in conditional
jumps using the relaxed forms, which is surprising. Based on the
discussion on IRC, I did a llvm/clang release build with -O0 for
OPTIMIZER and -O0 -mno-relax-all.

Result is:

-O0 (-mrelax-all)  68m27.893s 254m19.920s 1m45.720s
-O0 -mno-relax-all 68m6.640s  253m31.880s 1m45.310s

E.g. -mrelax-all is either harmful or not improving enough to give a
measurable improvement.

Can someone else try to reproduce this?

I've also did some testing with a worst case assembler file doing lots
of jumps that can all be relaxed. In that case, -mno-relax-all was 25%.
If the above observation can be confirmed, I believe there is no good
reason to keep -mrelax-all.

Joerg

> E.g. -mrelax-all is either harmful or not improving enough to give a
> measurable improvement.
> 
> Can someone else try to reproduce this?
I can. I tried these builds on tmpfs so as to give -mno-relax-all an
smaller advantaged for less IO:

--enable-optimized --with-optimize-option=-O0
real	3m38.568s
user	24m15.097s
sys	1m33.550s

clang is 69661804 bytes

 --enable-optimized --with-optimize-option="-O0 -mno-relax-all"

real	3m38.902s
user	24m18.423s
sys	1m34.074s

clang is 68805740 bytes

and on disk

 --enable-optimized --with-optimize-option="-O0 -g"

real	4m59.298s
user	29m22.683s
sys	2m24.325

clang is 686194963 bytes

 --enable-optimized --with-optimize-option="-O0 -g -mno-relax-all"
real	4m58.735s
user	29m34.127s
sys	2m25.734s

clang is 685305227 bytes
> I've also did some testing with a worst case assembler file doing lots
> of jumps that can all be relaxed. In that case, -mno-relax-all was 25%.
> If the above observation can be confirmed, I believe there is no good
> reason to keep -mrelax-all.
To the best of my knowledge relaxation is more expensive on ELF than on
Mach-O since there are less opportunities on Mach-O. Given that, I think
it is save to conclude that we can drop relax-all (and that I should
figure out what is going on with the size of debug binaries on ELF).
> Joerg
Cheers,
Rafael

>> 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.

Cheers,
Rafael