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Re: Question about GCC's standard dependent optimization


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



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