Doh! ENOPATCH.
-----Original Message-----
From: Roger Sayle <ro...@nextmovesoftware.com>
Sent: 19 August 2021 16:59
To: 'GCC Patches' <gcc-patches@gcc.gnu.org>
Subject: [x86_64 PATCH] Tweak -Os costs for scalar-to-vector pass.
Back in June I briefly mentioned in one of my gcc-patches posts that a
change that should have always reduced code size, would mysteriously
occasionally result in slightly larger code (according to CSiBE):
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573233.html
Investigating further, the cause turns out to be that x86_64's
scalar-to-vector (stv) pass is relying on poor estimates of the size
costs/benefits. This patch tweaks the backend's compute_convert_gain method
to provide slightly more accurate values when compiling with -Os.
Compilation without -Os is (should be) unaffected. And for completeness,
I'll mention that the stv pass is a net win for code size so it's much
better to improve its heuristics than simply gate the pass on
!optimize_for_size.
The net effect of this change is to save 1399 bytes on the CSiBE code size
benchmark when compiling with -Os.
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures.
Ok for mainline?
2021-08-19 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386-features.c (compute_convert_gain): Provide
more accurate values for CONST_INT, when optimizing for size.
* config/i386/i386.c (COSTS_N_BYTES): Move definition from here...
* config/i386/i386.h (COSTS_N_BYTES): to here.
Roger
--
diff --git a/gcc/config/i386/i386-features.c b/gcc/config/i386/i386-features.c
index d9c6652..cdae3dc 100644
--- a/gcc/config/i386/i386-features.c
+++ b/gcc/config/i386/i386-features.c
@@ -610,12 +610,31 @@ general_scalar_chain::compute_convert_gain ()
case CONST_INT:
if (REG_P (dst))
- /* DImode can be immediate for TARGET_64BIT and SImode always. */
- igain += m * COSTS_N_INSNS (1);
+ {
+ if (optimize_insn_for_size_p ())
+ {
+ /* xor (2 bytes) vs. xorps (3 bytes). */
+ if (src == const0_rtx)
+ igain -= COSTS_N_BYTES (1);
+ /* movdi_internal vs. movv2di_internal. */
+ /* => mov (5 bytes) vs. movaps (7 bytes). */
+ else if (x86_64_immediate_operand (src, SImode))
+ igain -= COSTS_N_BYTES (2);
+ }
+ else
+ {
+ /* DImode can be immediate for TARGET_64BIT
+ and SImode always. */
+ igain += m * COSTS_N_INSNS (1);
+ igain -= vector_const_cost (src);
+ }
+ }
else if (MEM_P (dst))
- igain += (m * ix86_cost->int_store[2]
- - ix86_cost->sse_store[sse_cost_idx]);
- igain -= vector_const_cost (src);
+ {
+ igain += (m * ix86_cost->int_store[2]
+ - ix86_cost->sse_store[sse_cost_idx]);
+ igain -= vector_const_cost (src);
+ }
break;
default:
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 4d4ab6a..5abf2a6 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -19982,8 +19982,6 @@ ix86_division_cost (const struct processor_costs *cost,
return cost->divide[MODE_INDEX (mode)];
}
-#define COSTS_N_BYTES(N) ((N) * 2)
-
/* Return cost of shift in MODE.
If CONSTANT_OP1 is true, the op1 value is known and set in OP1_VAL.
AND_IN_OP1 specify in op1 is result of and and SHIFT_AND_TRUNCATE
diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h
index 21fe51b..edbfcaf 100644
--- a/gcc/config/i386/i386.h
+++ b/gcc/config/i386/i386.h
@@ -88,6 +88,11 @@ struct stringop_algs
} size [MAX_STRINGOP_ALGS];
};
+/* Analog of COSTS_N_INSNS when optimizing for size. */
+#ifndef COSTS_N_BYTES
+#define COSTS_N_BYTES(N) ((N) * 2)
+#endif
+
/* Define the specific costs for a given cpu. NB: hard_register is used
by TARGET_REGISTER_MOVE_COST and TARGET_MEMORY_MOVE_COST to compute
hard register move costs by register allocator. Relative costs of
