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, 41 insertions, 0 deletions

diff --git a/gcc/df-core.cc b/gcc/df-core.cc
index a901b84..e3a56bf 100644
--- a/gcc/df-core.cc
+++ b/gcc/df-core.cc
@@ -2009,6 +2009,47 @@ df_reg_used (rtx_insn *insn, rtx reg)
   return df_find_use (insn, reg) != NULL;
 }
 
+/* If REG has a single definition, return its known value, otherwise return
+   null.  */
+
+rtx
+df_find_single_def_src (rtx reg)
+{
+  rtx src = NULL_RTX;
+
+  /* Don't look through unbounded number of single definition REG copies,
+     there might be loops for sources with uninitialized variables.  */
+  for (int cnt = 0; cnt < 128; cnt++)
+    {
+      df_ref adef = DF_REG_DEF_CHAIN (REGNO (reg));
+      if (adef == NULL || DF_REF_NEXT_REG (adef) != NULL
+	  || DF_REF_IS_ARTIFICIAL (adef)
+	  || (DF_REF_FLAGS (adef)
+	      & (DF_REF_PARTIAL | DF_REF_CONDITIONAL)))
+	return NULL_RTX;
+
+      rtx set = single_set (DF_REF_INSN (adef));
+      if (set == NULL || !rtx_equal_p (SET_DEST (set), reg))
+	return NULL_RTX;
+
+      rtx note = find_reg_equal_equiv_note (DF_REF_INSN (adef));
+      if (note && function_invariant_p (XEXP (note, 0)))
+	return XEXP (note, 0);
+      src = SET_SRC (set);
+
+      if (REG_P (src))
+	{
+	  reg = src;
+	  continue;
+	}
+      break;
+    }
+  if (!function_invariant_p (src))
+    return NULL_RTX;
+
+  return src;
+}
+
 
 /*----------------------------------------------------------------------------
    Debugging and printing functions.