[builtins] Write __divmoddi4/__divmodsi4 in terms __udivmod instead of __div and multiply.

[craig.topper]

Summary

Previously we calculating the remainder by multiplying the
quotient and divisor and subtracting from the dividend.

__udivmod can calculate the remainder while calculating the
quotient. We just need to correct the sign afterward.

[MaskRay]

compiler-rt/lib/builtins/divmodsi4.c

19–28

Just bits_in_word? m1 is not used.

[MaskRay]

compiler-rt/lib/builtins/divmodsi4.c

19–28

ok, m1 means minus 1...

compiler-rt/lib/builtins/divmoddi4.c

 //
 //===----------------------------------------------------------------------===//
 
 #include "int_lib.h"
 
 // Returns: a / b, *rem = a % b
 
 COMPILER_RT_ABI di_int __divmoddi4(di_int a, di_int b, di_int *rem) {
-  di_int d = __divdi3(a, b);
-  *rem = a - (d * b);
-  return d;
+  const int bits_in_dword_m1 = (int)(sizeof(di_int) * CHAR_BIT) - 1;
+  di_int s_a = a >> bits_in_dword_m1;                   // s_a = a < 0 ? -1 : 0
+  di_int s_b = b >> bits_in_dword_m1;                   // s_b = b < 0 ? -1 : 0
+  a = (a ^ s_a) - s_a;                                  // negate if s_a == -1
+  b = (b ^ s_b) - s_b;                                  // negate if s_b == -1
+  s_b ^= s_a;                                           // sign of quotient
+  du_int r;
+  di_int q = (__udivmoddi4(a, b, &r) ^ s_b) - s_b;      // negate if s_b == -1
+  *rem = (r ^ s_a) - s_a;                               // negate if s_a == -1
+  return q;
 }

compiler-rt/lib/builtins/divmodsi4.c

 //
 //===----------------------------------------------------------------------===//
 
 #include "int_lib.h"
 
 // Returns: a / b, *rem = a % b
 
 COMPILER_RT_ABI si_int __divmodsi4(si_int a, si_int b, si_int *rem) {
-  si_int d = __divsi3(a, b);
-  *rem = a - (d * b);
-  return d;
+  const int bits_in_word_m1 = (int)(sizeof(si_int) * CHAR_BIT) - 1;
+  si_int s_a = a >> bits_in_word_m1;                    // s_a = a < 0 ? -1 : 0
+  si_int s_b = b >> bits_in_word_m1;                    // s_b = b < 0 ? -1 : 0
+  a = (a ^ s_a) - s_a;                                  // negate if s_a == -1
+  b = (b ^ s_b) - s_b;                                  // negate if s_b == -1
+  s_b ^= s_a;                                           // sign of quotient
+  su_int r;
+  si_int q = (__udivmodsi4(a, b, &r) ^ s_b) - s_b;      // negate if s_b == -1
+  *rem = (r ^ s_a) - s_a;                               // negate if s_a == -1
+  return q;
 }