Fix the false positive of `SECP_64BIT_ASM

SpriteOvO commented at 10:50 am on April 15, 2022: contributor

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

 0src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r15' in 'asm'
 1   28 | __asm__ __volatile__(
 2      | ^
 3src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r14' in 'asm'
 4src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r13' in 'asm'
 5src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r12' in 'asm'
 6src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r11' in 'asm'
 7src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r10' in 'asm'
 8src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r9' in 'asm'
 9src/field_5x52_asm_impl.h:28:1: error: unknown register name '%r8' in 'asm'
10src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rdx' in 'asm'
11src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rcx' in 'asm'
12src/field_5x52_asm_impl.h:28:1: error: unknown register name '%rax' in 'asm'
13src/field_5x52_asm_impl.h:28:1: error: output number 0 not directly addressable
14src/field_5x52_asm_impl.h: In function 'secp256k1_fe_sqr':
15src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r15' in 'asm'
16  298 | __asm__ __volatile__(
17      | ^
18src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r14' in 'asm'
19src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r13' in 'asm'
20src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r12' in 'asm'
21src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r11' in 'asm'
22src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r10' in 'asm'
23src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r9' in 'asm'
24src/field_5x52_asm_impl.h:298:1: error: unknown register name '%r8' in 'asm'
25src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rdx' in 'asm'
26src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rcx' in 'asm'
27src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rbx' in 'asm'
28src/field_5x52_asm_impl.h:298:1: error: unknown register name '%rax' in 'asm'
29src/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:

0AC_DEFUN([SECP_64BIT_ASM_CHECK],[
1AC_MSG_CHECKING(for x86_64 assembly availability)
2AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
3  #include <stdint.h>]],[[
4  __asm__ __volatile__("this is obviously wrong");
5  ]])],[has_64bit_asm=yes],[has_64bit_asm=no])
6AC_MSG_RESULT([$has_64bit_asm])
7])

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.

Fix the false positive of `SECP_64BIT_ASM_CHECK` 7efc9835a9

real-or-random commented at 1:23 pm on April 15, 2022: contributor

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.

SpriteOvO commented at 2:11 pm on April 15, 2022: contributor

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).

real-or-random approved

real-or-random commented at 10:42 pm on April 15, 2022: contributor

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

real-or-random merged this on Apr 16, 2022

real-or-random closed this on Apr 16, 2022

SpriteOvO cross-referenced this on Apr 16, 2022 from issue addpkg: libsecp256k1 by SpriteOvO

laanwj commented at 6:25 pm on April 21, 2022: member

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.

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Fix the false positive of SECP_64BIT_ASM_CHECK #1104

Fix the false positive of `SECP_64BIT_ASM_CHECK` #1104