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This moves ARM over to the asm-generic/unaligned.h header. This has the
benefit of better code generated especially for ARMv7 on gcc 4.7+
compilers.
As Arnd Bergmann, points out: The asm-generic version uses the "struct"
version for native-endian unaligned access and the "byteshift" version
for the opposite endianess. The current ARM version however uses the
"byteshift" implementation for both.
Thanks to Nicolas Pitre for the excellent analysis:
Test case:
int foo (int *x) { return get_unaligned(x); }
long long bar (long long *x) { return get_unaligned(x); }
With the current ARM version:
foo:
ldrb r3, [r0, #2] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 2B], MEM[(const u8 *)x_1(D) + 2B]
ldrb r1, [r0, #1] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 1B], MEM[(const u8 *)x_1(D) + 1B]
ldrb r2, [r0, #0] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D)], MEM[(const u8 *)x_1(D)]
mov r3, r3, asl #16 @ tmp154, MEM[(const u8 *)x_1(D) + 2B],
ldrb r0, [r0, #3] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 3B], MEM[(const u8 *)x_1(D) + 3B]
orr r3, r3, r1, asl #8 @, tmp155, tmp154, MEM[(const u8 *)x_1(D) + 1B],
orr r3, r3, r2 @ tmp157, tmp155, MEM[(const u8 *)x_1(D)]
orr r0, r3, r0, asl #24 @,, tmp157, MEM[(const u8 *)x_1(D) + 3B],
bx lr @
bar:
stmfd sp!, {r4, r5, r6, r7} @,
mov r2, #0 @ tmp184,
ldrb r5, [r0, #6] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 6B], MEM[(const u8 *)x_1(D) + 6B]
ldrb r4, [r0, #5] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 5B], MEM[(const u8 *)x_1(D) + 5B]
ldrb ip, [r0, #2] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 2B], MEM[(const u8 *)x_1(D) + 2B]
ldrb r1, [r0, #4] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 4B], MEM[(const u8 *)x_1(D) + 4B]
mov r5, r5, asl #16 @ tmp175, MEM[(const u8 *)x_1(D) + 6B],
ldrb r7, [r0, #1] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 1B], MEM[(const u8 *)x_1(D) + 1B]
orr r5, r5, r4, asl #8 @, tmp176, tmp175, MEM[(const u8 *)x_1(D) + 5B],
ldrb r6, [r0, #7] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 7B], MEM[(const u8 *)x_1(D) + 7B]
orr r5, r5, r1 @ tmp178, tmp176, MEM[(const u8 *)x_1(D) + 4B]
ldrb r4, [r0, #0] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D)], MEM[(const u8 *)x_1(D)]
mov ip, ip, asl #16 @ tmp188, MEM[(const u8 *)x_1(D) + 2B],
ldrb r1, [r0, #3] @ zero_extendqisi2 @ MEM[(const u8 *)x_1(D) + 3B], MEM[(const u8 *)x_1(D) + 3B]
orr ip, ip, r7, asl #8 @, tmp189, tmp188, MEM[(const u8 *)x_1(D) + 1B],
orr r3, r5, r6, asl #24 @,, tmp178, MEM[(const u8 *)x_1(D) + 7B],
orr ip, ip, r4 @ tmp191, tmp189, MEM[(const u8 *)x_1(D)]
orr ip, ip, r1, asl #24 @, tmp194, tmp191, MEM[(const u8 *)x_1(D) + 3B],
mov r1, r3 @,
orr r0, r2, ip @ tmp171, tmp184, tmp194
ldmfd sp!, {r4, r5, r6, r7}
bx lr
In both cases the code is slightly suboptimal. One may wonder why
wasting r2 with the constant 0 in the second case for example. And all
the mov's could be folded in subsequent orr's, etc.
Now with the asm-generic version:
foo:
ldr r0, [r0, #0] @ unaligned @,* x
bx lr @
bar:
mov r3, r0 @ x, x
ldr r0, [r0, #0] @ unaligned @,* x
ldr r1, [r3, #4] @ unaligned @,
bx lr @
This is way better of course, but only because this was compiled for
ARMv7. In this case the compiler knows that the hardware can do
unaligned word access. This isn't that obvious for foo(), but if we
remove the get_unaligned() from bar as follows:
long long bar (long long *x) {return *x; }
then the resulting code is:
bar:
ldmia r0, {r0, r1} @ x,,
bx lr @
So this proves that the presumed aligned vs unaligned cases does have
influence on the instructions the compiler may use and that the above
unaligned code results are not just an accident.
Still... this isn't fully conclusive without at least looking at the
resulting assembly fron a pre ARMv6 compilation. Let's see with an
ARMv5 target:
foo:
ldrb r3, [r0, #0] @ zero_extendqisi2 @ tmp139,* x
ldrb r1, [r0, #1] @ zero_extendqisi2 @ tmp140,
ldrb r2, [r0, #2] @ zero_extendqisi2 @ tmp143,
ldrb r0, [r0, #3] @ zero_extendqisi2 @ tmp146,
orr r3, r3, r1, asl #8 @, tmp142, tmp139, tmp140,
orr r3, r3, r2, asl #16 @, tmp145, tmp142, tmp143,
orr r0, r3, r0, asl #24 @,, tmp145, tmp146,
bx lr @
bar:
stmfd sp!, {r4, r5, r6, r7} @,
ldrb r2, [r0, #0] @ zero_extendqisi2 @ tmp139,* x
ldrb r7, [r0, #1] @ zero_extendqisi2 @ tmp140,
ldrb r3, [r0, #4] @ zero_extendqisi2 @ tmp149,
ldrb r6, [r0, #5] @ zero_extendqisi2 @ tmp150,
ldrb r5, [r0, #2] @ zero_extendqisi2 @ tmp143,
ldrb r4, [r0, #6] @ zero_extendqisi2 @ tmp153,
ldrb r1, [r0, #7] @ zero_extendqisi2 @ tmp156,
ldrb ip, [r0, #3] @ zero_extendqisi2 @ tmp146,
orr r2, r2, r7, asl #8 @, tmp142, tmp139, tmp140,
orr r3, r3, r6, asl #8 @, tmp152, tmp149, tmp150,
orr r2, r2, r5, asl #16 @, tmp145, tmp142, tmp143,
orr r3, r3, r4, asl #16 @, tmp155, tmp152, tmp153,
orr r0, r2, ip, asl #24 @,, tmp145, tmp146,
orr r1, r3, r1, asl #24 @,, tmp155, tmp156,
ldmfd sp!, {r4, r5, r6, r7}
bx lr
Compared to the initial results, this is really nicely optimized and I
couldn't do much better if I were to hand code it myself.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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