https://gcc.gnu.org/g:ed8cd42d138fa048e0c0eff1ea28b39f5abe1c29
commit r15-6836-ged8cd42d138fa048e0c0eff1ea28b39f5abe1c29
Author: Maciej W. Rozycki <ma...@orcam.me.uk>
Date: Sun Jan 12 16:48:54 2025 +0000
Alpha: Fix a block move pessimisation with zero-extension after LDWU
For the BWX case we have a pessimisation in `alpha_expand_block_move'
for HImode loads where we place the data loaded into a HImode register
as well, therefore losing information that indeed the data loaded has
already been zero-extended to the full DImode width of the register.
Later on when we store this data in QImode quantities into an unaligned
destination, we zero-extend it again for the purpose of right-shifting,
such as with the test case included producing code at `-O2' as follows:
ldah $2,unaligned_src_hi($29) !gprelhigh
lda $1,unaligned_src_hi($2) !gprellow
ldwu $6,unaligned_src_hi($2) !gprellow
ldwu $5,2($1)
ldwu $4,4($1)
bis $31,$31,$31
zapnot $6,3,$3 # Redundant!
ldbu $7,6($1)
zapnot $5,3,$2 # Redundant!
stb $6,0($16)
zapnot $4,3,$1 # Redundant!
stb $5,2($16)
srl $3,8,$3
stb $4,4($16)
srl $2,8,$2
stb $3,1($16)
srl $1,8,$1
stb $2,3($16)
stb $1,5($16)
stb $7,6($16)
The non-BWX case is unaffected, because there we use byte insertion, so
we don't care that data is held in a HImode register.
Address this by making the holding RTX a HImode subreg of the original
DImode register, which the RTL passes can then see through and eliminate
the zero-extension where otherwise required, resulting in this shortened
code:
ldah $2,unaligned_src_hi($29) !gprelhigh
lda $1,unaligned_src_hi($2) !gprellow
ldwu $4,unaligned_src_hi($2) !gprellow
ldwu $3,2($1)
ldwu $2,4($1)
bis $31,$31,$31
srl $4,8,$6
ldbu $1,6($1)
srl $3,8,$5
stb $4,0($16)
stb $6,1($16)
srl $2,8,$4
stb $3,2($16)
stb $5,3($16)
stb $2,4($16)
stb $4,5($16)
stb $1,6($16)
While at it reformat the enclosing do-while statement according to the
GNU Coding Standards, observing that in this case it does not obfuscate
the change owing to the odd original indentation.
gcc/
* config/alpha/alpha.cc (alpha_expand_block_move): Use a HImode
subreg of a DImode register to hold data from an aligned HImode
load.
Diff:
---
gcc/config/alpha/alpha.cc | 17 +++++++++++------
.../gcc.target/alpha/memcpy-hi-unaligned-dst.c | 16 ++++++++++++++++
2 files changed, 27 insertions(+), 6 deletions(-)
diff --git a/gcc/config/alpha/alpha.cc b/gcc/config/alpha/alpha.cc
index 8ec9e8c5d399..6965ece16d0b 100644
--- a/gcc/config/alpha/alpha.cc
+++ b/gcc/config/alpha/alpha.cc
@@ -3999,14 +3999,19 @@ alpha_expand_block_move (rtx operands[])
if (bytes >= 2)
{
if (src_align >= 16)
- {
- do {
- data_regs[nregs++] = tmp = gen_reg_rtx (HImode);
- emit_move_insn (tmp, adjust_address (orig_src, HImode, ofs));
+ do
+ {
+ tmp = gen_reg_rtx (DImode);
+ emit_move_insn (tmp,
+ expand_simple_unop (DImode, SET,
+ adjust_address (orig_src,
+ HImode, ofs),
+ NULL_RTX, 1));
+ data_regs[nregs++] = gen_rtx_SUBREG (HImode, tmp, 0);
bytes -= 2;
ofs += 2;
- } while (bytes >= 2);
- }
+ }
+ while (bytes >= 2);
else if (! TARGET_BWX)
{
data_regs[nregs++] = tmp = gen_reg_rtx (HImode);
diff --git a/gcc/testsuite/gcc.target/alpha/memcpy-hi-unaligned-dst.c
b/gcc/testsuite/gcc.target/alpha/memcpy-hi-unaligned-dst.c
new file mode 100644
index 000000000000..4e3c02f5b906
--- /dev/null
+++ b/gcc/testsuite/gcc.target/alpha/memcpy-hi-unaligned-dst.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-mbwx" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" } } */
+
+unsigned short unaligned_src_hi[4];
+
+void
+memcpy_unaligned_dst_hi (void *dst)
+{
+ __builtin_memcpy (dst, unaligned_src_hi, 7);
+}
+
+/* { dg-final { scan-assembler-times "\\sldwu\\s" 3 } } */
+/* { dg-final { scan-assembler-times "\\sldbu\\s" 1 } } */
+/* { dg-final { scan-assembler-times "\\sstb\\s" 7 } } */
+/* { dg-final { scan-assembler-not "\\szapnot\\s" } } */
