build: detect arm32 assembly by default #1752

1. 
   Raimo33 commented at 5:29 pm on September 19, 2025: none

   This PR addresses issue #1751 by adding a call to check_arm32_assembly() by default, matching the current behavior with check_x86_64_assembly().

   This would result in speedup on field_10x26_impl.h on default builds. For example, currently, the Bitcoin Core reference implementation compiles libsecp256k1 with default options, leading to unoptimal builds.

   This change could help address https://github.com/bitcoin/bitcoin/issues/32832 partially, considering the flamegraph shows that ecdsa_verify takes 90% of IBD time.

   Benchmark	Avg(us) OFF	Avg(us) arm32	Improvement (%)
   ecdsa_verify	379.0	322.0	15.0
   ecdsa_sign	184.0	170.0	7.6
   ec_keygen	160.0	145.0	9.4
   ecdh	382.0	332.0	13.1
   schnorrsig_sign	162.0	148.0	8.6
   schnorrsig_verify	380.0	323.0	15.0
   ellswift_encode	109.0	95.1	12.7
   ellswift_decode	60.2	50.8	15.6
   ellswift_keygen	268.0	240.0	10.4
   ellswift_ecdh	395.0	343.0	13.2

2. Raimo33 force-pushed on Sep 19, 2025
3. Raimo33 force-pushed on Sep 19, 2025
4. 
   hebasto commented at 8:28 pm on September 19, 2025: member

   This would result in speedup on field_10x26_impl.h on default builds.

   Please provide benchmarks to support this statement.

5. Raimo33 force-pushed on Sep 20, 2025
6. Raimo33 force-pushed on Sep 20, 2025
7. 
   Raimo33 commented at 2:01 pm on September 22, 2025: none

   Please provide benchmarks to support this statement.

   will do. I’m buying a raspberry PI right now. I reckon if the benchmarks don’t show improvements we should delete field_10x26_arm.s entirely

8. 
   hebasto commented at 1:15 pm on September 25, 2025: member

   Perhaps convert this to a draft while the CI is red?
9. Raimo33 marked this as a draft on Sep 25, 2025
10. 
    Raimo33 commented at 4:00 pm on October 28, 2025: none

    Please provide benchmarks to support this statement.

    I’ve ran benchmarks on my raspberry pi 4. Here are the results.

    Optional features: assembly ………………………. OFF

     0Benchmark                     ,    Min(us)    ,    Avg(us)    ,    Max(us)    
     1
     2ecdsa_verify                  ,   378.0       ,   379.0       ,   379.0    
     3ecdsa_sign                    ,   184.0       ,   184.0       ,   185.0    
     4ec_keygen                     ,   160.0       ,   160.0       ,   160.0    
     5ecdh                          ,   382.0       ,   382.0       ,   383.0    
     6schnorrsig_sign               ,   162.0       ,   162.0       ,   162.0    
     7schnorrsig_verify             ,   380.0       ,   380.0       ,   381.0    
     8ellswift_encode               ,   109.0       ,   109.0       ,   109.0    
     9ellswift_decode               ,    60.1       ,    60.2       ,    60.3    
    10ellswift_keygen               ,   268.0       ,   268.0       ,   268.0    
    11ellswift_ecdh                 ,   395.0       ,   395.0       ,   395.0
    

    Optional features: assembly ………………………. arm32

     0Benchmark                     ,    Min(us)    ,    Avg(us)    ,    Max(us)    
     1
     2ecdsa_verify                  ,   322.0       ,   322.0       ,   322.0    
     3ecdsa_sign                    ,   170.0       ,   170.0       ,   170.0    
     4ec_keygen                     ,   145.0       ,   145.0       ,   145.0    
     5ecdh                          ,   332.0       ,   332.0       ,   333.0    
     6schnorrsig_sign               ,   148.0       ,   148.0       ,   149.0    
     7schnorrsig_verify             ,   323.0       ,   323.0       ,   324.0    
     8ellswift_encode               ,    94.9       ,    95.1       ,    95.3    
     9ellswift_decode               ,    50.6       ,    50.8       ,    50.9    
    10ellswift_keygen               ,   239.0       ,   240.0       ,   240.0    
    11ellswift_ecdh                 ,   343.0       ,   343.0       ,   343.0
    

11. 
    hebasto commented at 4:18 pm on October 28, 2025: member

    I’ve ran benchmarks on my raspberry pi 4. Here are the results.

    Which compiler did you use?

12. 
    Raimo33 commented at 4:25 pm on October 28, 2025: none

    Which compiler did you use?

    I used the option -DCMAKE_TOOLCHAIN_FILE=./cmake/arm-linux-gnueabihf.toolchain.cmake

    0C compiler ............................ GNU 13.3.0, /usr/bin/arm-linux-gnueabihf-gcc
    

13. 
    hebasto commented at 4:56 pm on October 28, 2025: member

    Which compiler did you use?

    I used the option -DCMAKE_TOOLCHAIN_FILE=./cmake/arm-linux-gnueabihf.toolchain.cmake

    0C compiler ............................ GNU 13.3.0, /usr/bin/arm-linux-gnueabihf-gcc
    

    This workflow is for cross-compiling. Did you try to build natively on your RPi?

14. build: remove assembly detection when explicitly disabled 18ea93ebdb
15. build: detect arm32 assembly by default 7d52166fd5
16. build: remove experimental warning for arm32 assembly 557bad20da
17. Raimo33 force-pushed on Oct 28, 2025
18. 
    Raimo33 commented at 5:56 pm on October 28, 2025: none

    Did you try to build natively on your RPi?

    I get:

    0/home/pi/secp256k1/src/asm/field_10x26_arm.s:875: Error: selected processor does not support `ubfx r2,r3,#0,#22' in ARM mode
    1/home/pi/secp256k1/src/asm/field_10x26_arm.s:880: Error: selected processor does not support `movw r14,field_R1<<4' in ARM mode
    

    when building natively on my RPI. the compiler is /usr/libexec/gcc/arm-linux-gnueabihf/14/ Apparently that’s because RPIs build for armv6 architectures, not armv7…

    and our field_10x26_arm.s is only compatible with armv7

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