At some past instance Linus Trovalds wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
> commit a84a79e4d3 upstream.
>
> The size is always valid, but variable-length arrays generate worse code
> for no good reason (unless the function happens to be inlined and the
> compiler sees the length for the simple constant it is).
>
> Also, there seems to be some code generation problem on POWER, where
> Henrik Bakken reports that register r28 can get corrupted under some
> subtle circumstances (interrupt happening at the wrong time?). That all
> indicates some seriously broken compiler issues, but since variable
> length arrays are bad regardless, there's little point in trying to
> chase it down.
>
> "Just don't do that, then".
Since then any use of "variable length arrays" has become blasphemous.
Even in perfectly good, beautiful, perfectly safe code like the one
below where the variable length arrays are only used as a sizeof()
parameter, for type-safe dynamic structure allocations. GCC is not
executing any stack allocation code.
I have produced a small file which defines two functions main1(unsigned numdevs)
and main2(unsigned numdevs). main1 uses code as before with call to malloc
and main2 uses code as of after this patch. I compiled it as:
gcc -O2 -S see_asm.c
and here is what I get:
<see_asm.s>
main1:
.LFB7:
.cfi_startproc
mov %edi, %edi
leaq 4(%rdi,%rdi), %rdi
salq $3, %rdi
jmp malloc
.cfi_endproc
.LFE7:
.size main1, .-main1
.p2align 4,,15
.globl main2
.type main2, @function
main2:
.LFB8:
.cfi_startproc
mov %edi, %edi
addq $2, %rdi
salq $4, %rdi
jmp malloc
.cfi_endproc
.LFE8:
.size main2, .-main2
.section .text.startup,"ax",@progbits
.p2align 4,,15
</see_asm.s>
*Exact* same code !!!
So please seriously consider not accepting this patch and leave the
perfectly good code intact.
CC: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
5318a29c19