[CompilerRT] Fix 128-bit floating point division

[kpdev42]

Summary

Patch fixes number of iterations which should be 5 for given reciprocal precision, not 4.

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

[efriedma]

Can you give some context here? How did we end up with "4" in the first place, and how do you know "5" is sufficient for all 128-bit floats?

compiler-rt/lib/builtins/divtf3.c

 //
 //===----------------------------------------------------------------------===//
 
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
 
-#define NUMBER_OF_HALF_ITERATIONS 4
+#define NUMBER_OF_HALF_ITERATIONS 5
 #define NUMBER_OF_FULL_ITERATIONS 1
 
 #include "fp_div_impl.inc"
 
 COMPILER_RT_ABI fp_t __divtf3(fp_t a, fp_t b) { return __divXf3__(a, b); }
 
 #endif

compiler-rt/lib/builtins/fp_div_impl.inc

#endif
   // Add 2 to U_N due to final decrement.
 
 #if defined(SINGLE_PRECISION) && NUMBER_OF_HALF_ITERATIONS == 2 && NUMBER_OF_FULL_ITERATIONS == 1
 #define RECIPROCAL_PRECISION REP_C(74)
 #elif defined(SINGLE_PRECISION) && NUMBER_OF_HALF_ITERATIONS == 0 && NUMBER_OF_FULL_ITERATIONS == 3
 #define RECIPROCAL_PRECISION REP_C(10)
 #elif defined(DOUBLE_PRECISION) && NUMBER_OF_HALF_ITERATIONS == 3 && NUMBER_OF_FULL_ITERATIONS == 1
 #define RECIPROCAL_PRECISION REP_C(220)
-#elif defined(QUAD_PRECISION) && NUMBER_OF_HALF_ITERATIONS == 4 && NUMBER_OF_FULL_ITERATIONS == 1
+#elif defined(QUAD_PRECISION) && NUMBER_OF_HALF_ITERATIONS == 5 && NUMBER_OF_FULL_ITERATIONS == 1
 #define RECIPROCAL_PRECISION REP_C(13922)
 #else
 #error Invalid number of iterations
 #endif
 
   // Suppose 1/b - P * 2^-W < x < 1/b + P * 2^-W
   x_UQ0 -= RECIPROCAL_PRECISION;
   // Now 1/b - (2*P) * 2^-W < x < 1/b

compiler-rt/lib/builtins/int_util.h

#define REPEAT_2_TIMES(code_to_repeat)                                         \
   REPEAT_1_TIMES(code_to_repeat)                                               \
   code_to_repeat
 #define REPEAT_3_TIMES(code_to_repeat)                                         \
   REPEAT_2_TIMES(code_to_repeat)                                               \
   code_to_repeat
 #define REPEAT_4_TIMES(code_to_repeat)                                         \
   REPEAT_3_TIMES(code_to_repeat)                                               \
   code_to_repeat
+#define REPEAT_5_TIMES(code_to_repeat)                                         \
+  REPEAT_4_TIMES(code_to_repeat)                                               \
+  code_to_repeat
+
 
 #define REPEAT_N_TIMES_(N, code_to_repeat) REPEAT_##N##_TIMES(code_to_repeat)
 #define REPEAT_N_TIMES(N, code_to_repeat) REPEAT_N_TIMES_(N, code_to_repeat)
 
 #endif // INT_UTIL_H

compiler-rt/test/builtins/Unit/divtf3_test.c

if (test__divtf3(0x1.2345f6b77b7a8953365433abcdefp+234L,
                      UINT64_C(0x5e6fcb6b60044078)))
         return 1;
     if (test__divtf3(6.72420628622418701252535563464350521E-4932L,
                      2.L,
                      UINT64_C(0x0001000000000000),
                      UINT64_C(0)))
         return 1;
 
+    if (test__divtf3(-1L,
+                     -0.999999999999999999999999999999999904L,
+                     UINT64_C(0x1),
+                     UINT64_C(0x3fff000000000000)))
+        return 1;
 #else
     printf("skipped\n");
 
 #endif
     return 0;
 }