Commit 16d2a186 authored by Niels Möller's avatar Niels Möller

ARM memxor: Delay push of registers. Accidentally slowed down memxor3.

parent 993ae2d6
2013-02-19 Niels Möller <nisse@lysator.liu.se>
* armv7/memxor.asm (memxor): Software pipelining for the aligned case.
* armv7/memxor.asm (memxor): Software pipelining for the aligned
case. Runs at 6 cycles (0.5 cycles per byte). Delayed push of
registers until we know how many registers we need.
(memxor3): Use 3-way unrolling also for aligned memxor3.
Both loops benchmarked at 7 cycles (0.58 cycles per byte), but
memxor3 seems to have a strange dependency on instruction
alignment.
Runs at 8 cycles (0.67 cycles per byte)
2013-02-12 Niels Möller <nisse@lysator.liu.se>
......
......@@ -30,7 +30,7 @@ define(<DST>, <r0>)
define(<SRC>, <r1>)
define(<N>, <r2>)
define(<CNT>, <r6>)
define(<TNC>, <r7>)
define(<TNC>, <r12>)
.syntax unified
......@@ -43,10 +43,7 @@ define(<TNC>, <r7>)
.align 4
PROLOGUE(memxor)
cmp N, #0
beq .Lmemxor_ret
C FIXME: Delay push until we know how many registers we need.
push {r4,r5,r6,r7,r8,r10,r11,r14} C lr is the link register
beq .Lmemxor_done
cmp N, #7
bcs .Lmemxor_large
......@@ -54,21 +51,19 @@ PROLOGUE(memxor)
C Simple byte loop
.Lmemxor_bytes:
ldrb r3, [SRC], #+1
ldrb r4, [DST]
eor r3, r4
ldrb r12, [DST]
eor r3, r12
strb r3, [DST], #+1
subs N, #1
bne .Lmemxor_bytes
.Lmemxor_done:
pop {r4,r5,r6,r7,r8,r10,r11,r14}
.Lmemxor_ret:
bx lr
.Lmemxor_align_loop:
ldrb r3, [SRC], #+1
ldrb r4, [DST]
eor r3, r4
ldrb r12, [DST]
eor r3, r12
strb r3, [DST], #+1
sub N, #1
......@@ -79,7 +74,7 @@ PROLOGUE(memxor)
C We have at least 4 bytes left to do here.
sub N, #4
ands CNT, SRC, #3
ands r3, SRC, #3
beq .Lmemxor_same
C Different alignment case.
......@@ -93,7 +88,9 @@ PROLOGUE(memxor)
C With little-endian, we need to do
C DST[i] ^= (SRC[i] >> CNT) ^ (SRC[i+1] << TNC)
lsl CNT, #3
push {r4,r5,r6}
lsl CNT, r3, #3
bic SRC, #3
rsb TNC, CNT, #32
......@@ -120,12 +117,15 @@ PROLOGUE(memxor)
subs N, #8
bcs .Lmemxor_word_loop
adds N, #8
beq .Lmemxor_done
beq .Lmemxor_odd_done
C We have TNC/8 left-over bytes in r4, high end
lsr r4, CNT
ldr r3, [DST]
eor r3, r4
pop {r4,r5,r6}
C Store bytes, one by one.
.Lmemxor_leftover:
strb r3, [DST], #+1
......@@ -134,10 +134,14 @@ PROLOGUE(memxor)
subs TNC, #8
lsr r3, #8
bne .Lmemxor_leftover
b .Lmemxor_bytes
.Lmemxor_odd_done:
pop {r4,r5,r6}
bx lr
.Lmemxor_same:
push {r4,r5,r6,r7,r8,r10,r11,r14} C lr is the link register
subs N, #8
bcc .Lmemxor_same_end
......@@ -154,8 +158,9 @@ PROLOGUE(memxor)
ldmia r14!, {r6, r7, r8}
bcc .Lmemxor_same_wind_down
C 7 cycles per iteration, 0.58 cycles/byte
C Loopmixer could perhaps get it down to 6 cycles.
C 6 cycles per iteration, 0.50 cycles/byte. For this speed,
C loop starts at offset 0x11c in the object file.
.Lmemxor_same_loop:
C r10-r12 contains values to be stored at DST
C r6-r8 contains values read from r14, in advance
......@@ -188,16 +193,18 @@ PROLOGUE(memxor)
eor r3, r6
eor r4, r7
stmia DST!, {r3, r4}
pop {r4,r5,r6,r7,r8,r10,r11,r14}
beq .Lmemxor_done
b .Lmemxor_bytes
.Lmemxor_same_lt_8:
pop {r4,r5,r6,r7,r8,r10,r11,r14}
adds N, #4
bcc .Lmemxor_same_lt_4
ldr r3, [SRC], #+4
ldr r4, [DST]
eor r3, r4
ldr r12, [DST]
eor r3, r12
str r3, [DST], #+4
beq .Lmemxor_done
b .Lmemxor_bytes
......@@ -342,10 +349,12 @@ PROLOGUE(memxor3)
subs N, #8
bcc .Lmemxor3_aligned_word_end
C This loop runs at 7 cycles per iteration, but it seems to
C have a strange alignment requirement. For this speed, the
C loop started at offset 0x2ac in the object file, and all
C other offsets made it slower.
C This loop runs at 8 cycles per iteration. It has been
C observed running at only 7 cycles, for this speed, the loop
C started at offset 0x2ac in the object file.
C FIXME: consider software pipelining, similarly to the memxor
C loop.
.Lmemxor3_aligned_word_loop:
ldmdb AP!, {r4,r5,r6}
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment