build: detect and compile arm32 assembly #1751

1. 
   Raimo33 commented at 4:53 pm on September 19, 2025: none

   Currently, arm32 assembly gets only compiled if explicitly set via SECP256K1_ASM=arm32. When SECP256K1_ASM is set to AUTO, then the detection script only checks for x86 assembly. I propose adding check_arm32_assembly() also in the AUTO branch. see https://github.com/bitcoin/bitcoin/issues/32832#issuecomment-3268056766.

   This is the current snippet for reference:

    0set(SECP256K1_ASM "AUTO" CACHE STRING "Assembly to use: \"AUTO\", \"OFF\", \"x86_64\" or \"arm32\" (experimental). [default=AUTO]")
    1set_property(CACHE SECP256K1_ASM PROPERTY STRINGS "AUTO" "OFF" "x86_64" "arm32")
    2check_string_option_value(SECP256K1_ASM)
    3if(SECP256K1_ASM STREQUAL "arm32")
    4  enable_language(ASM)
    5  include(CheckArm32Assembly)
    6  check_arm32_assembly()
    7  if(HAVE_ARM32_ASM)
    8    add_compile_definitions(USE_EXTERNAL_ASM=1)
    9  else()
   10    message(FATAL_ERROR "ARM32 assembly requested but not available.")
   11  endif()
   12elseif(SECP256K1_ASM)
   13  include(CheckX86_64Assembly)
   14  check_x86_64_assembly()
   15  if(HAVE_X86_64_ASM)
   16    set(SECP256K1_ASM "x86_64")
   17    add_compile_definitions(USE_ASM_X86_64=1)
   18  elseif(SECP256K1_ASM STREQUAL "AUTO")
   19    set(SECP256K1_ASM "OFF")
   20  else()
   21    message(FATAL_ERROR "x86_64 assembly requested but not available.")
   22  endif()
   23endif()
   

2. 
   hebasto commented at 8:25 pm on September 19, 2025: member

   Currently, arm32 assembly gets only compiled if explicitly set via SECP256K1_ASM=arm32. When SECP256K1_ASM is set to AUTO, then the detection script only checks for x86 assembly. I propose adding check_arm32_assembly() also in the AUTO branch. see bitcoin/bitcoin#32832 (comment).

   The comment from @sipa that you’re referring to clearly states:

   because they were … unreviewed @Raimo33

   If this situation has changed and the assembly code has been reviewed, please leave references here. Until then, I don’t see any reason to bring this into Bitcoin Core.

3. 
   Raimo33 commented at 11:02 pm on September 19, 2025: none

   doesn’t the CI test secp functions? both in Bitcoin core and here.

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