[src/trunk]: src/common/lib/libc/arch/arm/gen arm bswap32: Improve the commen...

[dholland <dholland%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/6916fcce44d7
branches:  trunk
changeset: 1016989:6916fcce44d7
user:      dholland <dholland%NetBSD.org@localhost>
date:      Wed Dec 09 02:46:57 2020 +0000

description:
arm bswap32: Improve the comments showing the byte flow.

It's confusing to use 1-4 for bytes 1-4 and then 0 for literal zero,
so use a-d for bytes 1-4.

diffstat:

 common/lib/libc/arch/arm/gen/byte_swap_4.S |  28 ++++++++++++++--------------
 1 files changed, 14 insertions(+), 14 deletions(-)

diffs (44 lines):

diff -r 87688446e8f2 -r 6916fcce44d7 common/lib/libc/arch/arm/gen/byte_swap_4.S
--- a/common/lib/libc/arch/arm/gen/byte_swap_4.S        Wed Dec 09 00:43:48 2020 +0000
+++ b/common/lib/libc/arch/arm/gen/byte_swap_4.S        Wed Dec 09 02:46:57 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: byte_swap_4.S,v 1.7 2013/08/19 03:44:47 matt Exp $     */
+/*     $NetBSD: byte_swap_4.S,v 1.8 2020/12/09 02:46:57 dholland Exp $ */
 
 /*-
  * Copyright (c) 1997 The NetBSD Foundation, Inc.
@@ -40,21 +40,21 @@
 #ifdef _ARM_ARCH_6
        rev     r0, r0
 #elif !defined(__thumb__)
-       eor     r1, r0, r0, ror #16     /* 4.3.2.1 -> 42.31.42.31 */
-       bic     r1, r1, #0x00FF0000     /* 42.31.42.31 -> 42.0.42.31 */
-       mov     r0, r0, ror #8          /* 4.3.2.1 -> 1.4.3.2 */
-       eor     r0, r0, r1, lsr #8      /* 1.4.3.2 ^ 0.42.0.42 -> 1.2.3.4 */
+       eor     r1, r0, r0, ror #16     /* d.c.b.a -> db.ca.db.ca */
+       bic     r1, r1, #0x00FF0000     /* db.ca.db.ca -> db.0.db.ca */
+       mov     r0, r0, ror #8          /* d.c.b.a -> a.d.c.b */
+       eor     r0, r0, r1, lsr #8      /* a.d.c.b ^ 0.db.0.db -> a.b.c.d */
 #else
        movs    r3, #16
-       lsls    r1, r0, #8              /* 4.3.2.1 -> 3.2.1.0 /*
-       lsrs    r0, r0, #8              /* 4.3.2.1 -> 0.4.3.2 */
-       rors    r1, r3                  /* 3.2.1.0 -> 1.0.3.2 */
-       rors    r0, r3                  /* 0.4.3.2 -> 3.2.0.4 */
-       lsrs    r1, r1, #8              /* 1.0.3.2 -> 0.1.0.3 */
-       lsls    r1, r1, #8              /* 0.1.0.3 -> 1.0.3.0 */
-       lsls    r0, r0, #8              /* 3.2.0.4 -> 2.0.4.0 */
-       lsrs    r0, r0, #8              /* 2.0.4.0 -> 0.2.0.4 */
-       orrs    r0, r0, r1              /* 1.0.3.0 | 0.2.0.4 -> 1.2.3.4 */
+       lsls    r1, r0, #8              /* d.c.b.a -> c.b.a.0 /*
+       lsrs    r0, r0, #8              /* d.c.b.a -> 0.d.c.b */
+       rors    r1, r3                  /* c.b.a.0 -> a.0.c.b */
+       rors    r0, r3                  /* 0.d.c.b -> c.b.0.d */
+       lsrs    r1, r1, #8              /* a.0.c.b -> 0.a.0.c */
+       lsls    r1, r1, #8              /* 0.a.0.c -> a.0.c.0 */
+       lsls    r0, r0, #8              /* c.b.0.d -> b.0.d.0 */
+       lsrs    r0, r0, #8              /* b.0.d.0 -> 0.b.0.d */
+       orrs    r0, r0, r1              /* a.0.c.0 | 0.b.0.d -> a.b.c.d */
 #endif
        RET
 END(FUNC)