From 99a6c1065de2db04d0f56f4b2cc89acecf21b72e Mon Sep 17 00:00:00 2001
From: Roger Sayle <roger@nextmovesoftware.com>
Date: Tue, 24 Oct 2023 16:45:08 +0100
Subject: [PATCH] i386: Fine tune STV register conversion costs for -Os.
The eagle-eyed may have spotted that my recent testcases for DImode shifts
on x86_64 included -mno-stv in the dg-options. This is because the
Scalar-To-Vector (STV) pass currently transforms these shifts to use
SSE vector operations, producing larger code even with -Os. The issue
is that the compute_convert_gain currently underestimates the size of
instructions required for interunit moves, which is corrected with the
patch below.
For the simple test case:
unsigned long long shl1(unsigned long long x) { return x << 1; }
without this patch, GCC -m32 -Os -mavx2 currently generates:
shl1: push %ebp // 1 byte
mov %esp,%ebp // 2 bytes
vmovq 0x8(%ebp),%xmm0 // 5 bytes
pop %ebp // 1 byte
vpaddq %xmm0,%xmm0,%xmm0 // 4 bytes
vmovd %xmm0,%eax // 4 bytes
vpextrd $0x1,%xmm0,%edx // 6 bytes
ret // 1 byte = 24 bytes total
with this patch, we now generate the shorter
shl1: push %ebp // 1 byte
mov %esp,%ebp // 2 bytes
mov 0x8(%ebp),%eax // 3 bytes
mov 0xc(%ebp),%edx // 3 bytes
pop %ebp // 1 byte
add %eax,%eax // 2 bytes
adc %edx,%edx // 2 bytes
ret // 1 byte = 15 bytes total
Benchmarking using CSiBE, shows that this patch saves 1361 bytes
when compiling with -m32 -Os, and saves 172 bytes when compiling
with -Os.
2023-10-24 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386-features.cc (compute_convert_gain): Provide
more accurate values (sizes) for inter-unit moves with -Os.
---
gcc/config/i386/i386-features.cc | 32 +++++++++++++++++++++++++++-----
1 file changed, 27 insertions(+), 5 deletions(-)
diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc
index cead3970cfce..6fac67e9a285 100644
--- a/gcc/config/i386/i386-features.cc
+++ b/gcc/config/i386/i386-features.cc
@@ -752,11 +752,33 @@ general_scalar_chain::compute_convert_gain ()
fprintf (dump_file, " Instruction conversion gain: %d\n", gain);
/* Cost the integer to sse and sse to integer moves. */
- cost += n_sse_to_integer * ix86_cost->sse_to_integer;
- /* ??? integer_to_sse but we only have that in the RA cost table.
- Assume sse_to_integer/integer_to_sse are the same which they
- are at the moment. */
- cost += n_integer_to_sse * ix86_cost->sse_to_integer;
+ if (!optimize_function_for_size_p (cfun))
+ {
+ cost += n_sse_to_integer * ix86_cost->sse_to_integer;
+ /* ??? integer_to_sse but we only have that in the RA cost table.
+ Assume sse_to_integer/integer_to_sse are the same which they
+ are at the moment. */
+ cost += n_integer_to_sse * ix86_cost->sse_to_integer;
+ }
+ else if (TARGET_64BIT || smode == SImode)
+ {
+ cost += n_sse_to_integer * COSTS_N_BYTES (4);
+ cost += n_integer_to_sse * COSTS_N_BYTES (4);
+ }
+ else if (TARGET_SSE4_1)
+ {
+ /* vmovd (4 bytes) + vpextrd (6 bytes). */
+ cost += n_sse_to_integer * COSTS_N_BYTES (10);
+ /* vmovd (4 bytes) + vpinsrd (6 bytes). */
+ cost += n_integer_to_sse * COSTS_N_BYTES (10);
+ }
+ else
+ {
+ /* movd (4 bytes) + psrlq (5 bytes) + movd (4 bytes). */
+ cost += n_sse_to_integer * COSTS_N_BYTES (13);
+ /* movd (4 bytes) + movd (4 bytes) + unpckldq (4 bytes). */
+ cost += n_integer_to_sse * COSTS_N_BYTES (12);
+ }
if (dump_file)
fprintf (dump_file, " Registers conversion cost: %d\n", cost);
--
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