===================================================================
@@ -1999,6 +1999,23 @@ replace_ref (tree *expr, slsr_cand_t c)
update_stmt (c->cand_stmt);
}
+/* Return true if CAND_REF candidate C is a valid memory reference. */
+
+static bool
+valid_mem_ref_cand_p (slsr_cand_t c)
+{
+ if (TREE_CODE (TREE_OPERAND (c->stride, 1)) != INTEGER_CST)
+ return false;
+
+ struct mem_address addr
+ = { NULL_TREE, c->base_expr, TREE_OPERAND (c->stride, 0),
+ TREE_OPERAND (c->stride, 1), wide_int_to_tree (sizetype, c->index) };
+
+ return
+ valid_mem_ref_p (TYPE_MODE (c->cand_type), TYPE_ADDR_SPACE (c->cand_type),
+ &addr);
+}
+
/* Replace CAND_REF candidate C, each sibling of candidate C, and each
dependent of candidate C with an equivalent strength-reduced data
reference. */
@@ -2006,6 +2023,16 @@ replace_ref (tree *expr, slsr_cand_t c)
static void
replace_refs (slsr_cand_t c)
{
+ /* Replacing a chain of only 2 candidates which are valid memory references
+ is generally counter-productive because you cannot recoup the additional
+ calculation added in front of them. */
+ if (c->basis == 0
+ && c->dependent
+ && !lookup_cand (c->dependent)->dependent
+ && valid_mem_ref_cand_p (c)
+ && valid_mem_ref_cand_p (lookup_cand (c->dependent)))
+ return;
+
if (dump_file && (dump_flags & TDF_DETAILS))
{
fputs ("Replacing reference: ", dump_file);
Hi, an user reported that, for pairs of consecutive memory accesses, the SLSR pass can slightly pessimize the generated code at -O2 on the x86 architecture: struct x { int a[16]; int b[16]; }; void set (struct x *p, unsigned int n, int i) { p->a[n] = i; p->b[n] = i; } is compiled with SLSR enabled into: leaq (%rdi,%rsi,4), %rax movl %edx, (%rax) movl %edx, 64(%rax) which is slightly worse than the expected: movl %edx, (%rdi,%rsi,4) movl %edx, 64(%rdi,%rsi,4) The attached patch is a tentative fix which doesn't seem to break anything. Tested on x86_64-suse-linux, OK for the mainline? 2019-08-02 Eric Botcazou <ebotcazou@adacore.com> * gimple-ssa-strength-reduction.c (valid_mem_ref_cand_p): New function. (replace_ref): Do not replace a chain of only two candidates which are valid memory references. 2019-08-02 Eric Botcazou <ebotcazou@adacore.com> * gcc.dg/tree-ssa/slsr-42.c: New test.