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Date: Thu, 25 Jun 2020 17:50:42 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v2] Add big-endian support to ARM assembler memcpy

On Tue, Jan 21, 2020 at 10:52:15AM -0800, Andre McCurdy wrote:
> Allow the existing ARM assembler memcpy implementation to be used for
> both big and little endian targets.
> ---
> 
> Exactly the same changes as before but rebased to account for
> whitespace changes in the preceding patch to add Thumb2 support.
> 
>  COPYRIGHT                                |   2 +-
>  src/string/arm/{memcpy_le.S => memcpy.S} | 101 ++++++++++++++++++++++-
>  src/string/arm/memcpy.c                  |   3 -
>  3 files changed, 98 insertions(+), 8 deletions(-)
>  rename src/string/arm/{memcpy_le.S => memcpy.S} (82%)
>  delete mode 100644 src/string/arm/memcpy.c
> 
> diff --git a/COPYRIGHT b/COPYRIGHT
> index e6472371..d3edc2a2 100644
> --- a/COPYRIGHT
> +++ b/COPYRIGHT
> @@ -127,7 +127,7 @@ Copyright © 2017-2018 Arm Limited
>  and labelled as such in comments in the individual source files. All
>  have been licensed under extremely permissive terms.
>  
> -The ARM memcpy code (src/string/arm/memcpy_el.S) is Copyright © 2008
> +The ARM memcpy code (src/string/arm/memcpy.S) is Copyright © 2008
>  The Android Open Source Project and is licensed under a two-clause BSD
>  license. It was taken from Bionic libc, used on Android.
>  
> diff --git a/src/string/arm/memcpy_le.S b/src/string/arm/memcpy.S
> similarity index 82%
> rename from src/string/arm/memcpy_le.S
> rename to src/string/arm/memcpy.S
> index 7b35d305..869e3448 100644
> --- a/src/string/arm/memcpy_le.S
> +++ b/src/string/arm/memcpy.S
> @@ -1,5 +1,3 @@
> -#if !__ARMEB__
> -
>  /*
>   * Copyright (C) 2008 The Android Open Source Project
>   * All rights reserved.
> @@ -42,7 +40,7 @@
>   * code safely callable from thumb mode, adjusting the return
>   * instructions to be compatible with pre-thumb ARM cpus, removal of
>   * prefetch code that is not compatible with older cpus and support for
> - * building as thumb 2.
> + * building as thumb 2 and big-endian.
>   */
>  
>  .syntax unified
> @@ -227,24 +225,45 @@ non_congruent:
>  	 * becomes aligned to 32 bits (r5 = nb of words to copy for alignment)
>  	 */
>  	movs    r5, r5, lsl #31
> +
> +#if __ARMEB__
> +	movmi   r3, r3, ror #24
> +	strbmi	r3, [r0], #1
> +	movcs   r3, r3, ror #24
> +	strbcs	r3, [r0], #1
> +	movcs   r3, r3, ror #24
> +	strbcs	r3, [r0], #1
> +#else
>  	strbmi r3, [r0], #1
>  	movmi   r3, r3, lsr #8
>  	strbcs r3, [r0], #1
>  	movcs   r3, r3, lsr #8
>  	strbcs r3, [r0], #1
>  	movcs   r3, r3, lsr #8
> +#endif
>  
>  	cmp     r2, #4
>  	blo     partial_word_tail
>  
> +#if __ARMEB__
> +	mov	r3, r3, lsr r12
> +	mov	r3, r3, lsl r12
> +#endif
> +
>  	/* Align destination to 32 bytes (cache line boundary) */
>  1:      tst     r0, #0x1c
>  	beq     2f
>  	ldr     r5, [r1], #4
>  	sub     r2, r2, #4
> +#if __ARMEB__
> +	mov     r4, r5,                 lsr lr
> +	orr     r4, r4, r3
> +	mov     r3, r5,                 lsl r12
> +#else
>  	mov     r4, r5,                 lsl lr
>  	orr     r4, r4, r3
>  	mov     r3, r5,                 lsr r12
> +#endif

Am I missing something or are both cases identical here? That would
either indicate this is gratuitous or there's a bug here and they were
intended not to be the same.

> [...]
> @@ -350,9 +429,15 @@ less_than_thirtytwo:
>  
>  1:      ldr     r5, [r1], #4
>  	sub     r2, r2, #4
> +#if __ARMEB__
> +	mov     r4, r5,                 lsr lr
> +	orr     r4, r4, r3
> +	mov     r3,     r5,                     lsl r12
> +#else
>  	mov     r4, r5,                 lsl lr
>  	orr     r4, r4, r3
>  	mov     r3,     r5,                     lsr r12
> +#endif

And again here.

Rich

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