RP2350 RISC-V compilation fails due to invalid inline asm machine constraint - platform.h

[iCMDdev]

When trying to compile a program using LLVM (clang) for the RISC-V mode of RP2350, an error is thrown at /src/rp2350/pico_platform/include/pico/platform.h:249:31:

error: invalid output constraint '+l' in asm
247 | __force_inline static int32_t __mul_instruction(int32_t a, int32_t b) {
248 | #ifdef __riscv
249 |     __asm ("mul %0, %0, %1" : "+l" (a) : "l" (b) : );
    |                               `- error: invalid output constraint '+l' in asm
250 | #else
251 |     pico_default_asm ("muls %0, %1"     : "+l" (a) : "l" (b) : "cc");

Looking at GCC's machine constraints for RISC-V, there isn't any l constraint specific to RISC-V, and there's no general l constraint. This also goes for LLVM.

This leads me to believe this was a mistake, since l is valid on ARM and was previously used for the ARM compilation mode. I'm not sure why / how this compilation succeeds on GCC, even though l is an invalid constraint for RISC-V.

One option is to replace the l constraint for RISC-V with r like this:

247 | __force_inline static int32_t __mul_instruction(int32_t a, int32_t b) {
248 | #ifdef __riscv
249 |     __asm ("mul %0, %0, %1" : "+r" (a) : "r" (b) : );
250 | #else
251 |     pico_default_asm ("muls %0, %1"     : "+l" (a) : "l" (b) : "cc");

[kilograham]

It may fail on GCC but only if you actually use the function

[iCMDdev]

Makes sense, I might not have used it in my testing. Thanks for merging!

[kilograham]

Yeah; i mean i think it is the same on Clang - we do check Clang compilation; i imagine it may be related to swift bridge headers - in any case the fix is in