http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51933
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-01-21
15:24:26 UTC ---
Consider following 3 routines:
unsigned long
foo (unsigned short *s, int i)
{
unsigned short x = s[i];
if (x < 0x211)
return (unsigned char) x;
asm volatile ("");
return 6;
}
unsigned long
bar (unsigned short *s, int i)
{
unsigned short x = s[i];
if (x < 0x211)
return (unsigned char) x;
asm volatile ("");
return x;
}
unsigned long
baz (unsigned short *s, int i)
{
unsigned short x = s[i];
if (x < 0x211)
return x;
asm volatile ("");
return (unsigned char) x;
}
In foo ree doesn't optimize away the QImode -> DImode zero extension, because
merge_def_and_ext checks:
&& GET_MODE (SET_DEST (*sub_rtx)) == ext_src_mode
and in this case ext_src_mode is QImode, but SET_DEST has HImode.
In bar ree doesn't optimize it away either, first merge_def_and_ext_checks is
called with the QImode ext_src_mode (which doesn't match) and only afterwards
on the HImode -> DImode extension, which is optimized (that is correct).
In baz (which is the same as bar in #c0 testcase) unfortunately
merge_def_and_ext is first called on the HImode -> DImode extension, which is
optimized (that is fine) by turning the HImode load into a DImode zero_extend
load from HImode and removing the HImode -> DImode extension.
But then when make_defs_and_copies_lists is called on the QImode -> DImode
extension, we reach:
/* Initialize the work list. */
if (!get_defs (extend_insn, src_reg, &work_list))
{
VEC_free (rtx, heap, work_list);
/* The number of defs being equal to zero can only mean that all the
definitions have been previously merged. */
return 2;
}
and because the definition has been changed already. But nothing performs the
ext_src_mode check that used to be performed otherwise.
So we either need to do the src mode checking earlier when we haven't started
modifying insns (in add_removable_extension?), or we'd need to look even at the
newly added defs and see what mode they extend from.
