incorrectly rounded square root

Paul Zimmermann
Fri Jun 4 07:14:07 GMT 2021

       Hi Jeff,

> I figured the values were off when I had to hard-code them in my own
> test_sqrt.c but forgot to include that info in my note.
> Now, that said, using the code I attached earlier, I am seeing the exact
> values you are quoting above for glibc for the mxcsr register and the round
> is working.  Have your
> tried running that code?

yes it works as expected, but it doesn't work with Newlib's fenv.h and libm.a
(see below).

> The mxcsr values you are seeing that are different are not due to the
> fesetround code.  The code is shifting the round value 13 bits
> and for 3, that ends up being 0x6000.  It is masking mxcsr with 0xffff9fff
> first so when you start with 0x1fxx and end up with 0x7fxx, the code is
> doing what is supposed to do.
> The difference in values above is 0x20 (e.g. 0x7fa0 vs 0x7f80) which is a
> bit in the last 2 hex digits which isn't touched by the code logic.

here is how to reproduce the issue:

tar xf newlib-4.1.0.tar.gz
cd newlib-4.1.0
mkdir build
cd build
../configure --prefix=/tmp --disable-multilib --target=x86_64
make -j4
make install

$ cat test_sqrt_2.c
#include <stdio.h>
#include <math.h>
#include <fenv.h>

#ifdef NEWLIB
/* RedHat's libm claims:
   undefined reference to `__errno' in j1f/y1f */
int errno;
int* __errno () { return &errno; }

int main()
  char Rnd[4] = "NZUD";
  float x = 0x1.ff07fep+127f;
  float y;
  for (int i = 0; i < 4; i++)
    unsigned short cw;
    unsigned int mxcsr = 0;
    fesetround (rnd[i]);
    __asm__ volatile ("fnstcw %0" : "=m" (cw) : );
    __asm__ volatile ("stmxcsr %0" : "=m" (mxcsr) : );
    y = sqrtf (x);
    printf ("RND%c: %a cw=%u mxcsr=%u\n", Rnd[i], y, cw, mxcsr);

With GNU libc:
$ gcc -fno-builtin test_sqrt_2.c -lm
$ ./a.out
RNDN: 0x1.ff83fp+63 cw=895 mxcsr=8064
RNDZ: 0x1.ff83eep+63 cw=3967 mxcsr=32672
RNDU: 0x1.ff83fp+63 cw=2943 mxcsr=24480
RNDD: 0x1.ff83eep+63 cw=1919 mxcsr=16288

With Newlib:
$ gcc -I/tmp/x86_64/include -DNEWLIB -fno-builtin test_sqrt_2.c /tmp/libm.a
$ ./a.out 
RNDN: 0x1.ff83fp+63 cw=895 mxcsr=8064
RNDZ: 0x1.ff83fp+63 cw=3967 mxcsr=32640
RNDU: 0x1.ff83fp+63 cw=2943 mxcsr=24448
RNDD: 0x1.ff83fp+63 cw=1919 mxcsr=16256

Can you reproduce that on x86_64 Linux?


More information about the Newlib mailing list