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Re: arm memcpy of aligned data
- From: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>
- To: Mike Stump <mikestump at comcast dot net>, gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Mon, 15 Jun 2015 15:30:20 +0100
- Subject: Re: arm memcpy of aligned data
- Authentication-results: sourceware.org; auth=none
- References: <9FF08D9A-529E-4FDE-9193-4BD81EDD89E3 at comcast dot net> <55682C81 dot 9040709 at arm dot com> <55683C58 dot 20701 at arm dot com>
On 29/05/15 11:15, Kyrill Tkachov wrote:
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.
The patch passes bootstrap and testing ok and I've seen it
improve codegen in a few places in SPEC.
I've added a testcase all marked up.
Mike, I'll commit the attached patch in 24 hours unless somebody objects.
Thanks,
Kyrill
2015-06-15 Mike Stump <mikestump@comcast.net>
* config/arm/arm.c (arm_block_move_unaligned_straight):
Emit normal move instead of unaligned load when source or destination
are appropriately aligned.
2015-06-15 Mike Stump <mikestump@comcast.net>
Kyrylo Tkachov <kyrylo.tkachov@arm.com>
* gcc.target/arm/memcpy-aligned-1.c: New 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.
commit 77191f4224c8729d014a9150bd9364f95ff704b0
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date: Fri May 29 10:44:21 2015 +0100
[ARM] arm memcpy of aligned data
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 638d659..3a33c26 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -14283,7 +14283,10 @@ arm_block_move_unaligned_straight (rtx dstbase, rtx srcbase,
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;
}
@@ -14302,7 +14305,10 @@ arm_block_move_unaligned_straight (rtx dstbase, rtx srcbase,
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;
}
diff --git a/gcc/testsuite/gcc.target/arm/memcpy-aligned-1.c b/gcc/testsuite/gcc.target/arm/memcpy-aligned-1.c
new file mode 100644
index 0000000..852b391
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/memcpy-aligned-1.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -save-temps" } */
+
+void *memcpy (void *dest, const void *src, unsigned int n);
+
+void foo (char *dst, int i)
+{
+ memcpy (dst, &i, sizeof (i));
+}
+
+/* { dg-final { scan-assembler-times "str\t" 1 } } */
+/* { dg-final { scan-assembler-not "ldr\t" } } */