aarch64: Fix sve/acle/general/ldff1_8.c failures

gcc.target/aarch64/sve/acle/general/ldff1_8.c and
gcc.target/aarch64/sve/ptest_1.c were failing because the
aarch64 port was giving a zero (unknown) cost to instructions
that compute two results in parallel.  This was latent until
r15-1575-gea8061f46a30, which fixed rtl-ssa to treat zero costs
as unknown.

A long-standing todo here is to make insn_cost derive costs from md
information, rather than having to write a lot of matching code in
aarch64_rtx_costs.  But that's not something we can do for GCC 15.

This patch instead treats the cost of a PARALLEL as being the maximum
cost of its constituent sets.  I don't like this very much, since it
isn't really target-specific behaviour.  If it were stage 1, I'd be
trying to change pattern_cost instead.

gcc/
	* config/aarch64/aarch64.cc (aarch64_insn_cost): Give PARALLELs
	the same cost as the costliest SET.

1 files changed, 18 insertions, 1 deletions

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 16754fa..c1e4020 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -15889,7 +15889,24 @@ aarch64_insn_cost (rtx_insn *insn, bool speed)
 {
   if (rtx set = single_set (insn))
     return set_rtx_cost (set, speed);
-  return pattern_cost (PATTERN (insn), speed);
+
+  /* If the instruction does multiple sets in parallel, use the cost
+     of the most expensive set.  This copes with instructions that set
+     the flags to a useful value as a side effect.  */
+  rtx pat = PATTERN (insn);
+  if (GET_CODE (pat) == PARALLEL)
+    {
+      int max_cost = 0;
+      for (int i = 0; i < XVECLEN (pat, 0); ++i)
+	{
+	  rtx x = XVECEXP (pat, 0, i);
+	  if (GET_CODE (x) == SET)
+	    max_cost = std::max (max_cost, set_rtx_cost (x, speed));
+	}
+      return max_cost;
+    }
+
+  return pattern_cost (pat, speed);
 }
 
 /* Implement TARGET_INIT_BUILTINS.  */