math: add x86_64 remquol

Submitted by Alexander Monakov on Jan. 16, 2020, 9 p.m.

Details

Message ID 20200116210051.19494-1-amonakov@ispras.ru
State New
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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;
+}