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Re: [PATCH][GCC][mid-end] Allow larger copies when not slow_unaligned_access and no padding.


Hi Richard,

Thanks for the review!

The 07/23/2018 18:46, Richard Biener wrote:
> On July 23, 2018 7:01:23 PM GMT+02:00, Tamar Christina <tamar.christina@arm.com> wrote:
> >Hi All,
> >
> >This allows copy_blkmode_to_reg to perform larger copies when it is
> >safe to do so by calculating
> >the bitsize per iteration doing the maximum copy allowed that does not
> >read more
> >than the amount of bits left to copy.
> >
> >Strictly speaking, this copying is only done if:
> >
> >  1. the target supports fast unaligned access
> >  2. no padding is being used.
> >
> >This should avoid the issues of the first patch (PR85123) but still
> >work for targets that are safe
> >to do so.
> >
> >Original patch https://gcc.gnu.org/ml/gcc-patches/2017-11/msg01088.html
> >Previous respin
> >https://gcc.gnu.org/ml/gcc-patches/2018-04/msg00239.html
> >
> >
> >This produces for the copying of a 3 byte structure:
> >
> >fun3:
> >	adrp	x1, .LANCHOR0
> >	add	x1, x1, :lo12:.LANCHOR0
> >	mov	x0, 0
> >	sub	sp, sp, #16
> >	ldrh	w2, [x1, 16]
> >	ldrb	w1, [x1, 18]
> >	add	sp, sp, 16
> >	bfi	x0, x2, 0, 16
> >	bfi	x0, x1, 16, 8
> >	ret
> >
> >whereas before it was producing
> >
> >fun3:
> >	adrp	x0, .LANCHOR0
> >	add	x2, x0, :lo12:.LANCHOR0
> >	sub	sp, sp, #16
> >	ldrh	w1, [x0, #:lo12:.LANCHOR0]
> >	ldrb	w0, [x2, 2]
> >	strh	w1, [sp, 8]
> >	strb	w0, [sp, 10]
> >	ldr	w0, [sp, 8]
> >	add	sp, sp, 16
> >	ret
> >
> >Cross compiled and regtested on
> >  aarch64_be-none-elf
> >  armeb-none-eabi
> >and no issues
> >
> >Boostrapped and regtested
> > aarch64-none-linux-gnu
> > x86_64-pc-linux-gnu
> > powerpc64-unknown-linux-gnu
> > arm-none-linux-gnueabihf
> >
> >and found no issues.
> >
> >OK for trunk?
> 
> How does this affect store-to-load forwarding when the source is initialized piecewise? IMHO we should avoid larger loads but generate larger stores when possible. 
> 
> How do non-x86 architectures behave with respect to STLF? 
>

I should have made it more explicit in my cover letter, but this only covers reg to reg copies.
So the store-t-load forwarding shouldn't really come to play here, unless I'm missing something

The example in my patch shows that the loads from mem are mostly unaffected.

For x86 the change is also quite significant, e.g for a 5 byte struct load it used to generate

fun5:
	movl	foo5(%rip), %eax
	movl	%eax, %edi
	movzbl	%al, %edx
	movzbl	%ah, %eax
	movb	%al, %dh
	movzbl	foo5+2(%rip), %eax
	shrl	$24, %edi
	salq	$16, %rax
	movq	%rax, %rsi
	movzbl	%dil, %eax
	salq	$24, %rax
	movq	%rax, %rcx
	movq	%rdx, %rax
	movzbl	foo5+4(%rip), %edx
	orq	%rsi, %rax
	salq	$32, %rdx
	orq	%rcx, %rax
	orq	%rdx, %rax
	ret

instead of

fun5:
        movzbl  foo5+4(%rip), %eax
        salq    $32, %rax
        movq    %rax, %rdx
        movl    foo5(%rip), %eax
        orq     %rdx, %rax
        ret

so the loads themselves are unaffected.

Thanks,
Tamar
 
> Richard. 
> 
> >Thanks,
> >Tamar
> >
> >gcc/
> >2018-07-23  Tamar Christina  <tamar.christina@arm.com>
> >
> >	* expr.c (copy_blkmode_to_reg): Perform larger copies when safe.
> 

-- 


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