Jeff Garzik wrote:
Handling of the trailing byte in ata_sff_data_xfer() is suboptimal
bacause:
- it always initializes the padding buffer to 0 which is not really
needed in
both the read and write cases;
- it has to use memcpy() to transfer a single byte from/to the padding
buffer;
Have you looked at the assembly, before deciding it is suboptiomal?
I'm estimating the code itself, not what the compiler can do to fix it. :-)
gcc optimizes tiny arrays and structures quite well, and is well capable
of seeing one path where the initialization is clobbered without a
single read, and another code path where it is used.
The initialier just shouldn't have been there in the first place,
clobbered or not. And let's looks at what gcc gave me:
.L504:
.loc 1 727 0
testb $1, %bl #, buflen
jne .L511 #,
[...]
.L511:
.LBB635:
.loc 1 731 0
movl 8(%ebp), %eax # rw,
.loc 1 729 0
leal (%esi,%ebx), %ebx #, tmp72
.LVL440:
.loc 1 728 0
.LBB635:
.loc 1 731 0
movl 8(%ebp), %eax # rw,
.loc 1 729 0
leal (%esi,%ebx), %ebx #, tmp72
.LVL440:
.loc 1 728 0
movw $0, -14(%ebp) #, align_buf
.loc 1 731 0
testl %eax, %eax #
jne .L507 #,
.loc 1 732 0
movl -20(%ebp), %eax # data_addr, data_addr
call ioread16 #
movw %ax, -14(%ebp) # D.29224, align_buf
.LBB629:
.LBB630:
.loc 4 60 0
movzbl -14(%ebp), %eax #, tmp73
movb %al, -1(%ebx) # tmp73,
.L509:
.LBE630:
.LBE629:
.loc 1 738 0
addl $1, %edi #, words
jmp .L505 #
.L507:
.LBB631:
.LBB632:
.loc 4 60 0
movzbl -1(%ebx), %eax #, tmp74
.LBE632:
.LBE631:
.loc 1 736 0
movzwl -14(%ebp), %eax # align_buf, align_buf
call iowrite16 #
jmp .L509 #
As you can see, it happily assigned 0 to align_buf[0] at .LVL440,
regardless of the value of 'rw'.
As for memcpy, for small and/or constant values that is quite often a
compiler builtin. It is rarely useful, these days, to convert a memcpy() to a hand-rolled
version of same.
Here memcpy() just shouldn't have appeared in the first place. But indeed,
gcc did optimize it away.
Jeff
MBR, Sergei
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