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Remove explicit width suffixes from Arm bignum assembly

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Within the M-profile of the Arm architecture, some instructions
admit both a 16-bit and a 32-bit encoding. For those instructions,
some assemblers support the use of the .n (narrow) and .w (wide)
suffixes to force a choice of instruction encoding width.
Forcing the size of encodings may be useful to ensure alignment
of code, which can have a significant performance impact on some
microarchitectures.

It is for this reason that a previous commit introduced explicit
.w suffixes into what was believed to be M-profile only assembly
in library/bn_mul.h.

This change, however, introduced two issues:
- First, the assembly block in question is used also for Armv7-A
  systems, on which the .n/.w distinction is not meaningful
  (all instructions are 32-bit).
- Second, compiler support for .n/.w suffixes appears patchy,
  leading to compilation failures even when building for M-profile
  targets.

This commit removes the .w annotations in order to restore working
code, deferring controlled re-introduction for the sake of performance.

Fixes #6089.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2022-07-15 12:00:58 +01:00
parent 0b5f4950cc
commit 907a367b50
1 changed files with 9 additions and 9 deletions
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18
library/bn_mul.h
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@ -717,10 +717,10 @@
#define MULADDC_X1_CORE \
".p2align 2 \n\t" \
"ldr.w %[a], [%[in]], #4 \n\t" \
"ldr.w %[b], [%[acc]] \n\t" \
"ldr %[a], [%[in]], #4 \n\t" \
"ldr %[b], [%[acc]] \n\t" \
"umaal %[b], %[carry], %[scalar], %[a] \n\t" \
"str.w %[b], [%[acc]], #4 \n\t"
"str %[b], [%[acc]], #4 \n\t"
#define MULADDC_X1_STOP \
: [a] "=&r" (tmp_a), \
@ -751,14 +751,14 @@
* 2 cycles, while subsequent loads/stores are single-cycle. */
#define MULADDC_X2_CORE \
".p2align 2 \n\t" \
"ldr.w %[a0], [%[in]], #+8 \n\t" \
"ldr.w %[b0], [%[acc]], #+8 \n\t" \
"ldr.w %[a1], [%[in], #-4] \n\t" \
"ldr.w %[b1], [%[acc], #-4] \n\t" \
"ldr %[a0], [%[in]], #+8 \n\t" \
"ldr %[b0], [%[acc]], #+8 \n\t" \
"ldr %[a1], [%[in], #-4] \n\t" \
"ldr %[b1], [%[acc], #-4] \n\t" \
"umaal %[b0], %[carry], %[scalar], %[a0] \n\t" \
"umaal %[b1], %[carry], %[scalar], %[a1] \n\t" \
"str.w %[b0], [%[acc], #-8] \n\t" \
"str.w %[b1], [%[acc], #-4] \n\t"
"str %[b0], [%[acc], #-8] \n\t" \
"str %[b1], [%[acc], #-4] \n\t"
#define MULADDC_X2_STOP \
: [a0] "=&r" (tmp_a0), \