diff options
| -rw-r--r-- | src/math/x86_64/remquol.c | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/src/math/x86_64/remquol.c b/src/math/x86_64/remquol.c new file mode 100644 index 00000000..60eef089 --- /dev/null +++ b/src/math/x86_64/remquol.c @@ -0,0 +1,32 @@ +#include <math.h> + +long double remquol(long double x, long double y, int *quo) +{ + signed char *cx = (void *)&x, *cy = (void *)&y; + /* By ensuring that addresses of x and y cannot be discarded, + * this empty asm guides GCC into representing extraction of + * their sign bits as memory loads rather than making x and y + * not-address-taken internally and using bitfield operations, + * which in the end wouldn't work out, as extraction from FPU + * registers needs to go through memory anyway. This way GCC + * should manage to use incoming stack slots without spills. */ + __asm__ ("" :: "X"(cx), "X"(cy)); + + long double t = x; + unsigned fpsr; + do __asm__ ("fprem1; fnstsw %%ax" : "+t"(t), "=a"(fpsr) : "u"(y)); + while (fpsr & 0x400); + /* C0, C1, C3 flags in x87 status word carry low bits of quotient: + * 15 14 13 12 11 10 9 8 + * . C3 . . . C2 C1 C0 + * . b1 . . . 0 b0 b2 */ + unsigned char i = fpsr >> 8; + i = i>>4 | i<<4; + /* i[5:2] is now {b0 b2 ? b1}. Retrieve {0 b2 b1 b0} via + * in-register table lookup. */ + unsigned qbits = 0x7575313164642020 >> (i & 60); + qbits &= 7; + + *quo = (cx[9]^cy[9]) < 0 ? -qbits : qbits; + return t; +} |