DSE: Use the constant store source if possible

RTL DSE tracks redundant constant stores within a basic block.  When RTL
loop invariant motion hoists a constant initialization out of the loop
into a separate basic block, the constant store value becomes unknown
within the original basic block.  When recording store for RTL DSE, check
if the source register is set only once to a constant by a non-partial
unconditional load.  If yes, record the constant as the constant store
source.  It eliminates unrolled zero stores after memset 0 in a loop
where a vector register is used as the zero store source.

gcc/

	PR rtl-optimization/105638
	* df-core.cc (df_find_single_def_src): Moved and renamed from
	find_single_def_src in loop-iv.cc.  Change the argument to rtx
	and use rtx_equal_p.  Return null for partial or conditional
	defs.
	* df.h (df_find_single_def_src): New prototype.
	* dse.cc (record_store): Use the constant source if the source
	register is set only once.
	* loop-iv.cc (find_single_def_src): Moved to df-core.cc.
	(replace_single_def_regs): Replace find_single_def_src with
	df_find_single_def_src.

gcc/testsuite/

	PR rtl-optimization/105638
	* g++.target/i386/pr105638.C: New test.

1 files changed, 1 insertions, 0 deletions

diff --git a/gcc/df.h b/gcc/df.h
index bd32920..71e249a 100644
--- a/gcc/df.h
+++ b/gcc/df.h
@@ -991,6 +991,7 @@ extern df_ref df_find_def (rtx_insn *, rtx);
 extern bool df_reg_defined (rtx_insn *, rtx);
 extern df_ref df_find_use (rtx_insn *, rtx);
 extern bool df_reg_used (rtx_insn *, rtx);
+extern rtx df_find_single_def_src (rtx);
 extern void df_worklist_dataflow (struct dataflow *,bitmap, int *, int);
 extern void df_print_regset (FILE *file, const_bitmap r);
 extern void df_print_word_regset (FILE *file, const_bitmap r);