[LLVMdev] [MC] Removing relaxation control

[Joerg Sonnenberger]

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