Fix the false positive of `SECP_64BIT_ASM_CHECK` #1104

I'm trying to compile this project for RISC-V architecture, and I encountered errors:

src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r15' in 'asm'
   28 | __asm__ __volatile__(
      | ^
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r14' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r13' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r12' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r11' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r10' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r9' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r8' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rdx' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rcx' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rax' in 'asm'
src/field_5x52_asm_impl.h:28:1: error: output number 0 not directly addressable
src/field_5x52_asm_impl.h: In function 'secp256k1_fe_sqr':
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r15' in 'asm'
  298 | __asm__ __volatile__(
      | ^
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r14' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r13' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r12' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r11' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r10' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r9' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r8' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rdx' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rcx' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rbx' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rax' in 'asm'
src/field_5x52_asm_impl.h:298:1: error: output number 0 not directly addressable

After further investigation I found that for RISC-V, macro USE_ASM_X86_64 was defined unexpectedly, and checking for x86_64 assembly availability... yes appeared in the compilation log file, which means SECP_64BIT_ASM_CHECK was not working as expected.

For unknown reasons, AC_COMPILE_IFELSE does not check if __asm__ can be compiled, and an example can verify this point:

AC_DEFUN([SECP_64BIT_ASM_CHECK],[
AC_MSG_CHECKING(for x86_64 assembly availability)
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
  #include <stdint.h>]],[[
  __asm__ __volatile__("this is obviously wrong");
  ]])],[has_64bit_asm=yes],[has_64bit_asm=no])
AC_MSG_RESULT([$has_64bit_asm])
])

It always gives results: checking for x86_64 assembly availability... yes

After testing, replacing AC_COMPILE_IFELSE with AC_LINK_IFELSE can correctly check if __asm__ can be compiled and make the project able to compile for RISC-V.

Thanks, I guess we should just use AC_LINK_IFELSE but can you tell me what compiler/toolchain you were using and can you share the configure.log for a failed run? It seems strange to me that this fails only at the linking stage.

Thanks, I guess we should just use AC_LINK_IFELSE but can you tell me what compiler/toolchain you were using and can you share the configure.log for a failed run? It seems strange to me that this fails only at the linking stage.

I cloned the build script PKGBUILD from the official ArchLinux, changed the beginning arch=(x86_64) to arch=(riscv64), and ran archbuild (this will enter a clean chroot environment) to compile this project in QEMU.

I don't see configure.log, but config.log, is it?

config.log

and archbuild logs that might help

libsecp256k1-20220317+1581+ge0508ee-1-riscv64-prepare.log libsecp256k1-20220317+1581+ge0508ee-1-riscv64-build.log

I agree it's strange, I tried to get the reason from Google and Autoconf's source code, but no luck (I don't know much about m4 language).

Hm, yeah, so it seems that the risc gcc when invoked normally will error on the x86_64 asm at the compile stage. But for whatever reason, when invoked with -flto (which is done by Arch Linux), it will only error at the link stage... Let's not bother with this and just check linking.

ACK 7efc9835a977c6400f0f024f19fda47710151dc1

But for whatever reason, when invoked with -flto

Correct, assembly is embedded as text in the IR file and only parsed at link time when using flto. That's when the real compilation happens.