Improve new C23 checked arithmetic feature

libc/intrin/divmodti4.c

@@ -0,0 +1,63 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to division,     │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include "third_party/compiler_rt/int_lib.h"
+
+/**
+ * Divides 128-bit signed integers w/ remainder.
+ *
+ * @param a is numerator
+ * @param b is denominator
+ * @param opt_out_rem receives euclidean division remainder if not null
+ * @return quotient or result of division
+ * @note rounds towards zero
+ */
+COMPILER_RT_ABI ti_int __divmodti4(ti_int a, ti_int b, tu_int *opt_out_rem) {
+  int k;
+  tu_int r;
+  ti_int sa, sb, sq, sr, x, y, q;
+  k = sizeof(ti_int) * CHAR_BIT - 1;
+  if (b == -1 && a == ((ti_int)1 << k)) {
+    volatile int x = 0;
+    x = 1 / x;  // raise(SIGFPE)
+  }
+  sa = a >> k;                 // sa = a < 0 ? -1 : 0
+  sb = b >> k;                 // sb = b < 0 ? -1 : 0
+  x = (a ^ sa) - sa;           // negate if sa == -1
+  y = (b ^ sb) - sb;           // negate if sb == -1
+  sq = sa ^ sb;                // sign of quotient
+  sr = sa;                     // sign of remainder
+  q = __udivmodti4(x, y, &r);  // unsigned divide
+  q = (q ^ sq) - sq;           // fix quotient sign
+  r = (r ^ sr) - sr;           // fix remainder sign
+  if (opt_out_rem) *opt_out_rem = r;
+  return q;
+}
+
+/*
+
+  Intel Kabylake i9-9900 @ 3.10GHz Client Grade
+
+    idiv32                         l:         27𝑐          9𝑛𝑠
+    idiv64                         l:         27𝑐          9𝑛𝑠
+    divmodti4 small / small        l:         42𝑐         14𝑛𝑠
+    divmodti4 small / large        l:         14𝑐          5𝑛𝑠
+    divmodti4 large / small        l:         92𝑐         30𝑛𝑠
+    divmodti4 large / large        l:        209𝑐         68𝑛𝑠
+
+  Intel Kabylake i3-8100 @ 3.60GHz Client Grade
+
+    idiv32                         l:         51𝑐         14𝑛𝑠
+    idiv64                         l:         51𝑐         14𝑛𝑠
+    divmodti4 small / small        l:         83𝑐         23𝑛𝑠
+    divmodti4 small / large        l:         26𝑐          7𝑛𝑠
+    divmodti4 large / small        l:        175𝑐         48𝑛𝑠
+    divmodti4 large / large        l:        389𝑐        107𝑛𝑠
+
+*/