Hi!
Below is an attempt to fix up RTX costing P1 caused by r15-775
https://gcc.gnu.org/pipermail/gcc-patches/2024-May/thread.html#652446
@@ -21562,7 +21562,8 @@ ix86_rtx_costs (rtx x, machine_mode mode, int
outer_code_i, int opno,
if (x86_64_immediate_operand (x, VOIDmode))
*total = 0;
else
- *total = 1;
+ /* movabsq is slightly more expensive than a simple instruction. */
+ *total = COSTS_N_INSNS (1) + 1;
return true;
case CONST_DOUBLE:
change. In my understanding this was partially trying to workaround
weird code in pattern_cost, which uses
return cost > 0 ? cost : COSTS_N_INSNS (1);
That doesn't make sense to me. All costs smaller than COSTS_N_INSNS (1)
mean we need to have at least one instruction there which has the
COSTS_N_INSNS (1) minimal cost. So special casing just cost 0 for the
really cheap immediates which can be used pretty much everywhere but not
ones which have just tiny bit larger cost than that (1, 2 or 3) is just
weird.
So, the following patch changes that to MAX (COSTS_N_INSNS (1), cost)
which doesn't have this weird behavior where set_src_cost 0 is considered
more expensive than set_src_cost 1.
Note, pattern_cost isn't the only spot where costs are computed and normally
we often sum the subcosts of different parts of a pattern or just query
rtx costs of different parts of subexpressions, so the jump from
1 to 5 is quite significant.
Additionally, x86_64 doesn't have just 2 kinds of constants with different
costs, it has 3, signed 32-bit ones are the ones which can appear in
almost all instructions and so using cost of 0 for those looks best,
then unsigned 32-bit ones which can be done with still cheap movl
instruction (and I think some others too) and finally full 64-bit ones
which can be done only with a single movabsq instruction and are quite
costly both in instruction size and even more expensive to execute.
The following patch attempts to restore the behavior of GCC 14 with the
pattern_cost hunk fixed for the unsigned 32-bit ones and only keeps the
bigger cost for the 64-bit ones.
Bootstrapped/regtested on x86_64-linux and i686-linux, bootstraps/regtests
in progress on aarch64-linux, powerpc64le-linux and s390x-linux.
I don't have a setup to test this on SPEC though.
Your thoughts on this?
2025-04-02 Jakub Jelinek <ja...@redhat.com>
PR target/115910
* rtlanal.cc (pattern_cost): Return at least COSTS_N_INSNS (1)
rather than just COSTS_N_INTNS (1) for cost <= 0.
* config/i386/i386.cc (ix86_rtx_costs): Set *total to 1 for
TARGET_64BIT x86_64_zext_immediate_operand constants.
* gcc.target/i386/pr115910.c: New test.
--- gcc/rtlanal.cc.jj 2025-03-06 11:08:20.729230232 +0100
+++ gcc/rtlanal.cc 2025-04-02 12:08:33.327409772 +0200
@@ -5772,7 +5772,7 @@ pattern_cost (rtx pat, bool speed)
return 0;
cost = set_src_cost (SET_SRC (set), GET_MODE (SET_DEST (set)), speed);
- return cost > 0 ? cost : COSTS_N_INSNS (1);
+ return MAX (COSTS_N_INSNS (1), cost);
}
/* Calculate the cost of a single instruction. A return value of zero
--- gcc/config/i386/i386.cc.jj 2025-03-27 23:35:17.798315113 +0100
+++ gcc/config/i386/i386.cc 2025-04-02 12:14:25.522539997 +0200
@@ -21883,7 +21883,11 @@ ix86_rtx_costs (rtx x, machine_mode mode
case SYMBOL_REF:
if (x86_64_immediate_operand (x, VOIDmode))
*total = 0;
- else
+ else if (TARGET_64BIT && x86_64_zext_immediate_operand (x, VOIDmode))
+ /* Consider the zext constants slightly more expensive, as they
+ can't appear in most instructions. */
+ *total = 1;
+ else
/* movabsq is slightly more expensive than a simple instruction. */
*total = COSTS_N_INSNS (1) + 1;
return true;
--- gcc/testsuite/gcc.target/i386/pr115910.c.jj 2025-04-02 12:27:36.199606571
+0200
+++ gcc/testsuite/gcc.target/i386/pr115910.c 2025-04-02 12:34:12.025132993
+0200
@@ -0,0 +1,20 @@
+/* PR target/115910 */
+/* { dg-do compile { target { ! ia32 } } } */
+/* { dg-options "-O2 -march=x86-64 -mtune=generic -masm=att" } */
+/* { dg-final { scan-assembler-times {\timulq\t} 2 } } */
+/* { dg-final { scan-assembler-times {\tshrq\t\$33,} 2 } } */
+/* { dg-final { scan-assembler-not {\tsarl\t} } } */
+
+int
+foo (int x)
+{
+ if (x < 0)
+ __builtin_unreachable ();
+ return x / 3U;
+}
+
+int
+bar (int x)
+{
+ return x / 3U;
+}
Jakub
