[Bug target/36133] GCC creates suboptimal ASM : Code includes unneeded TST instructions

["gunnar at greyhound-data dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from gunnar at greyhound-data dot com  2008-06-04 09:29 -------
I want to add that this wrong behavior is partly related to the compile option
"-Os".

There are two causes where GCC generates unneeded TST instructions.
A) General arithmetic 
 lsr.l #1,D0 
 tst.l d0
 jbne ...

This tst instruction is unneeded as the LSR is setting the flags correctly
already.

B) subq.l #1,D1
  tst.l d1
  jbne ...

This unneeded TST is related to the compile option used.
If you compile the source with "-O2" then the second unneeded TST instructions
are not included in the source.

It seems to me that a general important optimizations step - which used to be
in  "Os" in GCC 2.9 was removed from "Os" causing GCC to generate worse code
now.

Can you please be so kind and correct this?

I believe that this issue is quite serious for the performance of the generated
code.

1st The unneeded TST instructions are increasing code size, which is important
in embedded environments.

2nd
There are case were the instruction which really did set the condition codes
correctly in the first place is far enough away from the conditional branch and
no CC trashing instruction in between them - so that the instruction fetcher
can 100% correctly predict the branch and fold it away completely. The unneeded
TST instruction makes branch folding impossible and requires the CPU to guess
the branch instead. This will cause a serious performance impact in case of
mispredicting the branch.
It should be clear that the unneeded TST instruction doas not only bloat the
code but the above mentioned conditions can serious degrade the performance as
well, depending on your used CPU of course.

In the light of this, wouldn't it might sense to increase the Severity of this
issue?


Regards
Gunnar


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36133