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

[Richard Biener]

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? 

Richard. 

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