[2/5] cris: For expanded movsi, don't match operands we know will be reloaded
Commit Message
In a session investigating unexpected fallout from a change, I
noticed reload needs one operand being a register to make an
informed decision. It can happen that there's just a constant
and a memory operand, as in:
(insn 668 667 42 104 (parallel [
(set (mem:SI (plus:SI (reg/v/f:SI 347 [ fs ])
(const_int 168 [0xa8])) \
[1 fs_126(D)->regs.cfa_how+0 S4 A8])
(const_int 2 [0x2]))
(clobber (reg:CC 19 dccr))
]) "<...>/gcc/libgcc/unwind-dw2.c":1121:21 22 {*movsi_internal}
(expr_list:REG_UNUSED (reg:CC 19 dccr)
(nil)))
This was helpfully created by combine. When this happens,
reload can't check for costs and preferred register classes,
(both operands will start with NO_REGS as the preferred class)
and will default to the constraints order in the insn in reload.
(Which also does its own temporary merge in find_reloads, but
that's a different story.) Better don't match the simple cases.
Beware that subregs have to be matched.
I'm doing this just for word_mode (SI) for now, but may repeat
this for the other valid modes as well. In particular, that
goes for DImode as I see the expanded movdi does *almost* this,
but uses register_operand instead of REG_S_P (from cris.h).
Using REG_S_P is the right choice here because register_operand
also matches (subreg (mem ...) ...) *until* reload is done.
By itself it's just a sub-0.1% performance win (coremark).
Also removing a stale comment.
gcc:
* config/cris/cris.md ("*movsi_internal<setcc><setnz><setnzvc>"):
Conditionalize on (sub-)register operands or operand 1 being 0.
---
gcc/config/cris/cris.md | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
@@ -583,9 +583,10 @@ (define_insn "*movsi_internal<setcc><setnz><setnzvc>"
(match_operand:SI 1 "general_operand"
"r,Q>,M,M, I,r, M,n,g,r,x, rQ>,x,gi"))
(clobber (reg:CC CRIS_CC0_REGNUM))]
- ;; Note that we prefer not to use the S alternative (if for some reason
- ;; it competes with others) above, but g matches S.
- ""
+ ;; Avoid matching insns we know must be reloaded. Without one
+ ;; operand being a (pseudo-)register, reload chooses
+ ;; reload-registers suboptimally.
+ "REG_S_P (operands[0]) || REG_S_P (operands[1]) || operands[1] == const0_rtx"
{
/* Better to have c-switch here; it is worth it to optimize the size of
move insns. The alternative would be to try to find more constraint