[gcc r14-4881] aarch64: Define TARGET_INSN_COST

[gcc r14-4881] aarch64: Define TARGET_INSN_COST

[Richard Sandiford]

https://gcc.gnu.org/g:21416caf221fae4351319ef8ca8d41c0234bdfa7

commit r14-4881-g21416caf221fae4351319ef8ca8d41c0234bdfa7
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Oct 24 11:01:52 2023 +0100

    aarch64: Define TARGET_INSN_COST
    
    This patch adds a bare-bones TARGET_INSN_COST.  See the comment
    in the patch for the rationale.
    
    Just to get a flavour for how much difference it makes, I tried
    compiling the testsuite with -Os -fno-schedule-insns{,2} and
    seeing what effect the patch had on the number of instructions.
    Very few tests changed, but all the changes were positive:
    
      Tests   Good    Bad   Delta    Best   Worst  Median
      =====   ====    ===   =====    ====   =====  ======
         19     19      0    -177     -52      -1      -4
    
    The change for -O2 was even smaller, but more mixed:
    
      Tests   Good    Bad   Delta    Best   Worst  Median
      =====   ====    ===   =====    ====   =====  ======
          6      3      3      -8      -9       6      -2
    
    There were no obvious effects on SPEC CPU2017.
    
    The patch is needed to avoid a regression with a later change.
    
    gcc/
            * config/aarch64/aarch64.cc (aarch64_insn_cost): New function.
            (TARGET_INSN_COST): Define.

Diff:
---
 gcc/config/aarch64/aarch64.cc | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index a28b66acf6a5..aec9b76ed8c1 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -15541,6 +15541,28 @@ aarch64_memory_move_cost (machine_mode mode, reg_class_t rclass_i, bool in)
 	  : aarch64_tune_params.memmov_cost.store_int);
 }
 
+/* Implement TARGET_INSN_COST.  We have the opportunity to do something
+   much more productive here, such as using insn attributes to cost things.
+   But we don't, not yet.
+
+   The main point of this current definition is to make calling insn_cost
+   on one instruction equivalent to calling seq_cost on a sequence that
+   contains only that instruction.  The default definition would instead
+   only look at SET_SRCs, ignoring SET_DESTs.
+
+   This ensures that, for example, storing a 128-bit zero vector is more
+   expensive than storing a 128-bit vector register.  A move of zero
+   into a 128-bit vector register followed by multiple stores of that
+   register is then cheaper than multiple stores of zero (which would
+   use STP of XZR).  This in turn allows STP Qs to be formed.  */
+static int
+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);
+}
+
 /* Implement TARGET_INIT_BUILTINS.  */
 static void
 aarch64_init_builtins ()
@@ -28399,6 +28421,9 @@ aarch64_libgcc_floating_mode_supported_p
 #undef TARGET_RTX_COSTS
 #define TARGET_RTX_COSTS aarch64_rtx_costs_wrapper
 
+#undef TARGET_INSN_COST
+#define TARGET_INSN_COST aarch64_insn_cost
+
 #undef TARGET_SCALAR_MODE_SUPPORTED_P
 #define TARGET_SCALAR_MODE_SUPPORTED_P aarch64_scalar_mode_supported_p