Re: Question about GCC's standard dependent optimization

[Jeff Law <law at redhat dot com>]

On 06/26/14 02:44, Bin.Cheng wrote:
> Hi,
> I ran into PR60947, in which GCC understands the return value of
> memset is the first argument passed in, according to standard, then
> does optimization like below:
>      mov    ip, sp
>      stmfd    sp!, {r4, r5, r6, r7, r8, r9, r10, fp, ip, lr, pc}
>      sub    fp, ip, #4
>      sub    sp, sp, #20
>      ldr    r8, [r0, #112]
>      add    r3, r8, #232
>      add    r4, r8, #328
> .L1064:
>      mov    r0, r3
>      mov    r1, #255
>      mov    r2, #8
>      bl    memset
>      add    r3, r0, #32   <----X
>      cmp    r3, r4
>      bne    .L1064
>
> For X insn, GCC takes advantage of standard by using the returned r0 directly.
>
> My question is, is it always safe for GCC to do such optimization?  Do
> we have an option to disable such standard dependent optimization?
Others have already answered this question.

FWIW, I just locally added the capability to track equivalences between 
the destination argument to the various mem* str* functions and their 
return value in DOM.  It triggers, but not terribly often.  I'll be 
looking to see if the additional equivalences actually enable any 
optimizations before going through the full bootstrap and test.

It doesn't help your testcase though since optimizer weakness you're 
showing is much later in the optimization pipeline.  I wonder if you 
could model this as an implicit copy in IRA and try to encourage IRA to 
tie the hard reg output from memset to the pseudo.


jeff