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
