"impossible constraint in 'asm'" error when adding single-precision hard-float 'sqrt'

I want to use the single-precision hard-float sqrt instruction as my FPU supports sqrt operation, and I made some changes to Newlib:

  1. Create a new C file named ef_sqrt.c under newlib/libm/machine/riscv . The content of the new file is
#include <math.h>

__ieee754_sqrtf(float x)
	asm ("fsqrt.s %0, %1" : "=f" (x) : "f" (x));
	return x;
  1. Modify the corresponding Makefile.am and Makefile.in under newlib/libm/machine/riscv to include ef_sqrt.c .

When I tried to build the toolchain using riscv-none-embed-gcc-xpack,I got the “impossible constraint in ‘asm’” error at the inline assembly line asm ("fsqrt.s %0, %1" : "=f" (x) : "f" (x)); in my new file.

Hi Remember,

I’m not sure about the impossible constraint in ‘asm’” error message but would this thread below help you?

I would guess that you have a multilib build, and the soft-float multilibs are giving you the impossible constraint error because they don’t have FP registers. The code needs to be conditional on the existance of FP registers.

By the way, Keith Packard recently posted patches upstream for fma and sqrt support. I would suggest using that code. See for instance

The forum software is doing something funny with URLs. It seems I have to switch into URL mode to add a link correctly. Sigh.


1 Like

Thank you. That solves my problem.