From 299c98578bda88c020a6d5b2c319c9e191a315d4 Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Tue, 11 Aug 2020 13:47:29 +0200 Subject: [PATCH] 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 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. --- gcc/expr.c | 6 ++++++ gcc/testsuite/gcc.target/i386/pr96539.c | 16 ++++++++++++++++ 2 files changed, 22 insertions(+) create mode 100644 gcc/testsuite/gcc.target/i386/pr96539.c 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; diff --git a/gcc/testsuite/gcc.target/i386/pr96539.c b/gcc/testsuite/gcc.target/i386/pr96539.c new file mode 100644 index 0000000..fc164f8 --- /dev/null +++ b/gcc/testsuite/gcc.target/i386/pr96539.c @@ -0,0 +1,16 @@ +/* PR rtl-optimization/96539 */ +/* { dg-do compile } * +/* { dg-options "-Os" } */ +/* { dg-final { scan-assembler-not "rep\[^\n\r]\*movs" } } */ + +struct A { int a, b, c, d, e, f; void *g, *h, *i, *j, *k, *l, *m; }; + +int bar (int a); +int baz (int a, int b, int c, void *p, struct A s); + +int +foo (int a, int b, int c, void *p, struct A s) +{ + bar (a); + return baz (a, b, c, p, s); +} -- 2.7.4