v1: https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563800.html
v1 -> v2: Allow (mem) -> (subreg) propagation only for single uses.
Boostrapped and regtested on x86_64-redhat-linux, ppc64le-redhat-linux
and s390x-redhat-linux. Ok for master?
Suppose we have:
(set (reg/v:TF 63) (mem/c:TF (reg/v:DI 62)))
(set (reg:FPRX2 66) (subreg:FPRX2 (reg/v:TF 63) 0))
It is clearly profitable to propagate the first insn into the second
one and get:
(set (reg:FPRX2 66) (mem/c:FPRX2 (reg/v:DI 62)))
fwprop actually manages to perform this, but doesn't think the result is
worth it, which results in unnecessary store/load sequences on s390.
Improve the situation by classifying SUBREG -> MEM changes as
profitable.
gcc/ChangeLog:
2021-01-15 Ilya Leoshkevich <i...@linux.ibm.com>
* fwprop.c (fwprop_propagation::classify_result): Allow
(subreg (mem)) simplifications.
---
gcc/fwprop.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)
diff --git a/gcc/fwprop.c b/gcc/fwprop.c
index eff8f7cc141..02d3d507cbc 100644
--- a/gcc/fwprop.c
+++ b/gcc/fwprop.c
@@ -176,7 +176,7 @@ namespace
static const uint16_t CONSTANT = FIRST_SPARE_RESULT << 1;
static const uint16_t PROFITABLE = FIRST_SPARE_RESULT << 2;
- fwprop_propagation (rtx_insn *, rtx, rtx);
+ fwprop_propagation (rtx_insn *, insn_info *, rtx, rtx);
bool changed_mem_p () const { return result_flags & CHANGED_MEM; }
bool folded_to_constants_p () const;
@@ -185,13 +185,18 @@ namespace
bool check_mem (int, rtx) final override;
void note_simplification (int, uint16_t, rtx, rtx) final override;
uint16_t classify_result (rtx, rtx);
+
+ private:
+ const bool single_use_p;
};
}
/* Prepare to replace FROM with TO in INSN. */
-fwprop_propagation::fwprop_propagation (rtx_insn *insn, rtx from, rtx to)
- : insn_propagation (insn, from, to)
+fwprop_propagation::fwprop_propagation (rtx_insn *insn, insn_info *def_insn,
+ rtx from, rtx to)
+ : insn_propagation (insn, from, to),
+ single_use_p (def_insn->num_uses () == 1)
{
should_check_mems = true;
should_note_simplifications = true;
@@ -262,6 +267,13 @@ fwprop_propagation::classify_result (rtx old_rtx, rtx
new_rtx)
&& GET_MODE (new_rtx) == GET_MODE_INNER (GET_MODE (from)))
return PROFITABLE;
+ /* Allow (subreg (mem)) -> (mem) simplifications. Do not allow propagation
+ of (mem)s into multiple uses, since those are not profitable, as well as
+ creating new (mem/v)s, since DCE will not remove the old ones. */
+ if (single_use_p && SUBREG_P (old_rtx) && MEM_P (new_rtx)
+ && !MEM_VOLATILE_P (new_rtx))
+ return PROFITABLE;
+
return 0;
}
@@ -363,7 +375,7 @@ try_fwprop_subst_note (insn_info *use_insn, insn_info
*def_insn,
rtx_insn *use_rtl = use_insn->rtl ();
insn_change_watermark watermark;
- fwprop_propagation prop (use_rtl, dest, src);
+ fwprop_propagation prop (use_rtl, def_insn, dest, src);
if (!prop.apply_to_rvalue (&XEXP (note, 0)))
{
if (dump_file && (dump_flags & TDF_DETAILS))
@@ -426,7 +438,7 @@ try_fwprop_subst_pattern (obstack_watermark &attempt,
insn_change &use_change,
rtx_insn *use_rtl = use_insn->rtl ();
insn_change_watermark watermark;
- fwprop_propagation prop (use_rtl, dest, src);
+ fwprop_propagation prop (use_rtl, def_insn, dest, src);
if (!prop.apply_to_pattern (loc))
{
if (dump_file && (dump_flags & TDF_DETAILS))
--
2.26.2
