http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57836
Bug ID: 57836
Summary: large constants evaluated inline
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: amodra at gmail dot com
On powerpc64 with -mcmodel=small -O1, this
int x;
void f1 (long long hx)
{
if (hx < 0x3ff0000000000000LL)
x++;
}
results in
.L.f1:
lis 9,0x3fef
ori 9,9,65535
sldi 9,9,32
oris 9,9,0xffff
ori 9,9,65535
cmpd 7,3,9
bgtlr 7
ld 9,.LC1@toc(2)
lwz 10,0(9)
addi 10,10,1
stw 10,0(9)
blr
The -mcmodel isn't significant, just there to make comparison with older
compilers easy. Prior to gcc-4.5 we generated
.L.f1:
ld 0,.LC0@toc(2)
cmpd 7,3,0
bgtlr 7
ld 9,.LC1@toc(2)
lwz 11,0(9)
addi 0,11,1
stw 0,0(9)
blr
Both pre- and post-4.5 compilers initially expand rtl using the constant load
from toc, but 4.5 and later substitute the constant in the first cse pass.
The problem stems from rtx cost for the load from toc being the same as the
inline constant form.
The costs are the same both pre- and post-4.5. The reason pre-4.5 does not
substitute the constant is related to this comment is cse.c:
/* Find cheapest and skip it for the next time. For items
of equal cost, use this order:
src_folded, src, src_eqv, src_related and hash table entry. */
Pre-4.5 src_folded is NULL, src is a mem, src_eqv the constant.
Post-4.5 src_folded is the constant, src is a mem, src_eqv the constant again.
Pre-4.5, rs6000.c lacked delegitimize_address. src_folded is set by fold_rtx,
which calls equiv_constant, which calls avoid_constant_pool_reference, which
calls targetm.delegitimize_address. When this is missing, we don't get to see
the constant..
