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[RFC:,3/2,v3] Don't expand a conditional move between identical sources

Message ID 1469008295-28884-3-git-send-email-james.greenhalgh@arm.com
State New
Headers show

Commit Message

James Greenhalgh July 20, 2016, 9:51 a.m. UTC 
Hi,

This patch adds a short-circuit to optabs.c for the case where both
source operands are identical (i.e. we would be assigning the same
value in both branches).

This can show up for the memory optimisation in noce_cmove_arith in ifcvt.c,
if both branches would load from the same address. This is an odd situation
to arrise. It showed up in my csibe runs, but I couldn't reproduce it in
a small test case.

Bootstrapped on x86_64-none-linux-gnu and aarch64-none-linux-gnu with no
issues.

OK?

Thanks,
James

---
2016-07-20  James Greenhalgh  <james.greenhalgh@arm.com>

	* optabs.c (emit_condiitonal_move): Short circuit for identical
	sources.
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Patch

diff --git a/gcc/optabs.c b/gcc/optabs.c
index 51e10e2..87b4f97 100644
--- a/gcc/optabs.c
+++ b/gcc/optabs.c
@@ -4214,6 +4214,17 @@  emit_conditional_move (rtx target, enum rtx_code code, rtx op0, rtx op1,
   enum insn_code icode;
   enum rtx_code reversed;
 
+  /* If the two source operands are identical, that's just a move.  */
+
+  if (rtx_equal_p (op2, op3))
+    {
+      if (!target)
+	target = gen_reg_rtx (mode);
+
+      emit_move_insn (target, op3);
+      return target;
+    }
+
   /* If one operand is constant, make it the second one.  Only do this
      if the other operand is not constant as well.  */