On 29/05/15 10:08, Kyrill Tkachov wrote:
Hi Mike,
On 28/05/15 22:15, Mike Stump wrote:
So, the arm memcpy code of aligned data isn’t as good as it can be.
void *memcpy(void *dest, const void *src, unsigned int n);
void foo(char *dst, int i) {
memcpy (dst, &i, sizeof (i));
}
generates horrible code, but, it we are willing to notice the src or the
destination are aligned, we can do much better:
$ ./cc1 -fschedule-fusion -fdump-tree-all-all -da -march=armv7ve
-mcpu=cortex-m4 -fomit-frame-pointer -quiet -O2 /tmp/t.c -o t.s
$ cat t.s
[ … ]
foo:
@ args = 0, pretend = 0, frame = 4
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
sub sp, sp, #4
str r1, [r0] @ unaligned
add sp, sp, #4
I think there's something to do with cpu tuning here as well.
That being said, I do think this is a good idea.
I'll give it a test.
Kyrill
For the code you've given compiled with -O2 -mcpu=cortex-a53 I get:
sub sp, sp, #8
mov r2, r0
add r3, sp, #8
str r1, [r3, #-4]!
ldr r0, [r3] @ unaligned
str r0, [r2] @ unaligned
add sp, sp, #8
@ sp needed
bx lr
whereas for -O2 -mcpu=cortex-a57 I get the much better:
sub sp, sp, #8
str r1, [r0] @ unaligned
add sp, sp, #8
@ sp needed
bx lr
Kyrill
Index: gcc/config/arm/arm.c
===================================================================
--- gcc/config/arm/arm.c (revision 223842)
+++ gcc/config/arm/arm.c (working copy)
@@ -14376,7 +14376,10 @@ arm_block_move_unaligned_straight (rtx d
srcoffset + j * UNITS_PER_WORD - src_autoinc);
mem = adjust_automodify_address (srcbase, SImode, addr,
srcoffset + j * UNITS_PER_WORD);
- emit_insn (gen_unaligned_loadsi (regs[j], mem));
+ if (src_aligned)
+ emit_move_insn (regs[j], mem);
+ else
+ emit_insn (gen_unaligned_loadsi (regs[j], mem));
}
srcoffset += words * UNITS_PER_WORD;
}
@@ -14395,7 +14398,10 @@ arm_block_move_unaligned_straight (rtx d
dstoffset + j * UNITS_PER_WORD - dst_autoinc);
mem = adjust_automodify_address (dstbase, SImode, addr,
dstoffset + j * UNITS_PER_WORD);
- emit_insn (gen_unaligned_storesi (mem, regs[j]));
+ if (dst_aligned)
+ emit_move_insn (mem, regs[j]);
+ else
+ emit_insn (gen_unaligned_storesi (mem, regs[j]));
}
dstoffset += words * UNITS_PER_WORD;
}
Ok?
Can someone spin this through an arm test suite run for me, I was doing this by
inspection and cross compile on a system with no arm bits. Bonus points if you
can check it in with the test case above marked up as appropriate.
