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RFC: RTL sharing between decls and instructions
- From: Richard Sandiford <richard at codesourcery dot com>
- To: gcc at gcc dot gnu dot org
- Date: Fri, 27 Jul 2007 22:25:28 +0100
- Subject: RFC: RTL sharing between decls and instructions
unshare_all_rtl used to unshare DECL_RTLs as well as expressions in the
instruction stream. That changed with:
I think that patch was in itself the right thing to do. However, in
anticipation of the old unshare_all_rtl behaviour, unshare_all_rtl_again
used to reset the used flags on all those DECL_RTLs. I think that's no
longer correct. unshare_all_rtl_again is used after reload, which may
have introduced invalid sharing by converting pseudo registers into
non-register expressions. These sharing problems exist between
DECL_RTLs and the instruction stream as well as between instructions
Thus I think the right thing now is to get unshare_all_rtl_again to
_set_ rather than reset the used flags on DECL_RTLs, so that if a
DECL_RTL mentions an ex-register, all references to that ex-register
in the instruction stream will be copied. This is what the old
unshare_all_decls did as a side effect.
Does that sound right? I've checked that this patch fixes the case
I'm seeing, but I'm not too familiar with this code, so I wanted to
check whether I was barking up the wrong tree.
Sorry in advance if this has been suggested before.
* emit-rtl.c (reset_used_decls): Rename to...
(set_used_decls): ...this. Set the used flag rather than clearing it.
(unshare_all_rtl_again): Update accordingly. Set flags on argument
DECL_RTLs rather than resetting them.
--- gcc/emit-rtl.c (revision 126993)
+++ gcc/emit-rtl.c (working copy)
@@ -167,7 +167,7 @@ #define first_label_num (cfun->emit->x_f
static rtx make_call_insn_raw (rtx);
static rtx change_address_1 (rtx, enum machine_mode, rtx, int);
-static void reset_used_decls (tree);
+static void set_used_decls (tree);
static void mark_label_nuses (rtx);
static hashval_t const_int_htab_hash (const void *);
static int const_int_htab_eq (const void *, const void *);
@@ -2160,11 +2160,11 @@ unshare_all_rtl_again (rtx insn)
/* Make sure that virtual stack slots are not shared. */
- reset_used_decls (DECL_INITIAL (cfun->decl));
+ set_used_decls (DECL_INITIAL (cfun->decl));
/* Make sure that virtual parameters are not shared. */
for (decl = DECL_ARGUMENTS (cfun->decl); decl; decl = TREE_CHAIN (decl))
- reset_used_flags (DECL_RTL (decl));
+ set_used_flags (DECL_RTL (decl));
@@ -2353,20 +2353,28 @@ unshare_all_rtl_in_chain (rtx insn)
/* Go through all virtual stack slots of a function and mark them as
- not shared. */
+ shared. We never replace the DECL_RTLs themselves with a copy,
+ but expressions mentioned into a DECL_RTL cannot be shared with
+ expressions in the instruction stream.
+ Note that reload may convert pseudo registers into memories in-place.
+ Pseudo registers are always shared, but MEMs never are. Thus if we
+ reset the used flags on MEMs in the instruction stream, we must set
+ them again on MEMs that appear in DECL_RTLs. */
-reset_used_decls (tree blk)
+set_used_decls (tree blk)
/* Mark decls. */
for (t = BLOCK_VARS (blk); t; t = TREE_CHAIN (t))
if (DECL_RTL_SET_P (t))
- reset_used_flags (DECL_RTL (t));
+ set_used_flags (DECL_RTL (t));
/* Now process sub-blocks. */
for (t = BLOCK_SUBBLOCKS (blk); t; t = TREE_CHAIN (t))
- reset_used_decls (t);
+ set_used_decls (t);
/* Mark ORIG as in use, and return a copy of it if it was already in use.