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

[ams at gcc dot gnu dot org]

------- Comment #7 from ams at gcc dot gnu dot org  2008-11-10 12:29 -------
(In reply to comment #4)
> 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.

This is NOT correct. LSL does write to the condition codes, but not all of it.
In particular, the bit involved in the not-equal test is not set.

This TST *is* required.

> 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.

Please check the code mode carefully. This TST is not related to the SUBQ
instruction. This TST is related to the LSL instruction - the TST is a branch
target.

The compiler has combined the initial and subsequent loop condition tests into
one test, thus saving space, as per the -Os requirements. In this case there is
an alternative, more speed efficient solution, but that is a separate problem,
and not a misuse of the TST instruction.

As you say, the compiler gets it right at -O2, so the machine description is
clearly correct, in this respect.


-- 

ams at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


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