http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55565
--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2013-01-10
17:46:39 UTC ---
I compared the code generated by trunk with the generated code in rev 190339
which broke the test. The trunk code is more optimal than when the test
"passed", so I suggest either removing the dg-final in the test, or modifying
the regex in the dg-final.
In the testcase we have something functionally equivalent to (where x is the
original function argument passed in f1):
if (huge + x > 0.0) {
if ((int)x > 0)
return 0.0;
...
} else
return x;
When the test passed, we used to generate:
.L17:
ld %r8,.LC0@toc(%r2)
ld %r10,.LC2@toc(%r2)
lfd %f13,0(%r8)
lfd %f0,0(%r10)
fadd %f1,%f1,%f13
fcmpu %cr7,%f1,%f0
bng %cr7,.L4 /* if (!(huge + x > 0.0)) return x; */
...
...
.L4: /* return x; */
std %r9,-8(%r1)
ori 2,2,0
lfd %f1,-8(%r1)
blr
Notice that since we perform the fadd onto %f1, we need to reload %f1 from
memory.
On trunk though, we keep x/%f1 and 0.0/%f0 around so we can return them
directly. We can return x/%f1 directly in one case, or copy 0.0/%f0 to the
return register (%f1) in the another case.
.L18:
ld %r8,.LC6@toc(%r2)
ld %r9,.LC1@toc(%r2)
lfd %f13,0(%r8)
lfd %f0,0(%r9)
fadd %f13,%f1,%f13
fcmpu %cr7,%f13,%f0
bnglr %cr7 /* if (!(huge + x > 0.0)) return x; */
cmpdi %cr7,%r10,0
fmr %f1,%f0
bgelr %cr7 /* if (huge+x>0.0 && (int)x >0) return 0.0 */
Bottom line, on trunk we avoid a branch and memory load/stores.
Testing a patch.
