expr: Optimize noop copies [PR96539]

At GIMPLE e.g. for __builtin_memmove we optimize away (to just the return
value) noop copies where src == dest, but at the RTL we don't, and as the
testcase shows, in some cases such copies can appear only at the RTL level
e.g. from trying to copy an aggregate by value argument to the same location
as it already has.  If the block move is expanded e.g. piecewise, we
actually manage to optimize it away, as the individual memory copies are
seen as noop moves, but if the target optabs are used, often the sequences
stay until final.

2020-08-11  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/96539
	* expr.c (emit_block_move_hints): Don't copy anything if x and y
	are the same and neither is MEM_VOLATILE_P.

	* gcc.target/i386/pr96539.c: New test.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/expr.c b/gcc/expr.c
index ebf0c9e..2406f90 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -1637,6 +1637,12 @@ emit_block_move_hints (rtx x, rtx y, rtx size, enum block_op_methods method,
   x = adjust_address (x, BLKmode, 0);
   y = adjust_address (y, BLKmode, 0);
 
+  /* If source and destination are the same, no need to copy anything.  */
+  if (rtx_equal_p (x, y)
+      && !MEM_VOLATILE_P (x)
+      && !MEM_VOLATILE_P (y))
+    return 0;
+
   /* Set MEM_SIZE as appropriate for this block copy.  The main place this
      can be incorrect is coming from __builtin_memcpy.  */
   poly_int64 const_size;