Fix the predefined exponent limit macros for the 16-bit IEEE format.

[rjmccall]

Summary

Fix the predefined exponent limit macros for the 16-bit IEEE format.

The magnitude range of normalized _Float16 is 2^-14 (~6e-5) to (2-2^-10)*2^15 (65504). You might think, then, that the code is correct to defne FLT16_MIN_EXP and FLT16_MAX_EXP to be -14 and 15 respectively. However, for some reason the C specification actually specifies a bias for these macros:

C11 5.2.4.2.2:

    - minimum negative integer such that FLT_RADIX raised to one less than
      that power is a normalized floating-point number, e_min:
        FLT_MIN_EXP
        DBL_MIN_EXP
        LDBL_MIN_EXP
  
    - maximum integer such that FLT_RADIX raised to one less than that
      power is a representable finite floating-point number, e_max:
        FLT_MAX_EXP
        DBL_MAX_EXP
        LDBL_MAX_EXP

FLT16_MIN_EXP and FLT16_MAX_EXP should clearly be biased the same way, and other compilers do in fact do so, as do our OpenCL headers for half.

Additionally, FLT16_MIN_10_EXP is just wrong.

[rjmccall]

r362183

clang/lib/Frontend/InitPreprocessor.cpp

                    "4.94065645841246544176568792868221e-324",
                    "1.92592994438723585305597794258492732e-34");
   int MantissaDigits = PickFP(Sem, 11, 24, 53, 64, 106, 113);
-  int Min10Exp = PickFP(Sem, -13, -37, -307, -4931, -291, -4931);
+  int Min10Exp = PickFP(Sem, -4, -37, -307, -4931, -291, -4931);
   int Max10Exp = PickFP(Sem, 4, 38, 308, 4932, 308, 4932);
-  int MinExp = PickFP(Sem, -14, -125, -1021, -16381, -968, -16381);
-  int MaxExp = PickFP(Sem, 15, 128, 1024, 16384, 1024, 16384);
+  int MinExp = PickFP(Sem, -13, -125, -1021, -16381, -968, -16381);
+  int MaxExp = PickFP(Sem, 16, 128, 1024, 16384, 1024, 16384);
   Min = PickFP(Sem, "6.103515625e-5", "1.17549435e-38", "2.2250738585072014e-308",
                "3.36210314311209350626e-4932",
                "2.00416836000897277799610805135016e-292",

clang/test/Headers/float16.c

 
 #ifndef FLT16_MIN_10_EXP
     #error "Macro FLT16_MIN_10_EXP is missing."
-#elif   FLT16_MIN_10_EXP > -13
+#elif   FLT16_MIN_10_EXP > -4
     #error "Macro FLT16_MIN_10_EXP is invalid."
 #endif
 

 
 #ifndef FLT16_MIN_EXP
     #error "Macro FLT16_MIN_EXP is missing."
-#elif   FLT16_MIN_EXP > -14
+#elif   FLT16_MIN_EXP > -13
     #error "Macro FLT16_MIN_EXP is invalid."
 #endif
 

 
 #ifndef FLT16_MAX_EXP
     #error "Macro FLT16_MAX_EXP is missing."
-#elif   FLT16_MAX_EXP < 15
+#elif   FLT16_MAX_EXP < 16
     #error "Macro FLT16_MAX_EXP is invalid."
 #endif
 

clang/test/Preprocessor/init.c

 // AARCH64:#define __FLT16_HAS_QUIET_NAN__ 1
 // AARCH64:#define __FLT16_MANT_DIG__ 11
 // AARCH64:#define __FLT16_MAX_10_EXP__ 4
-// AARCH64:#define __FLT16_MAX_EXP__ 15
+// AARCH64:#define __FLT16_MAX_EXP__ 16
 // AARCH64:#define __FLT16_MAX__ 6.5504e+4F16
-// AARCH64:#define __FLT16_MIN_10_EXP__ (-13)
-// AARCH64:#define __FLT16_MIN_EXP__ (-14)
+// AARCH64:#define __FLT16_MIN_10_EXP__ (-4)
+// AARCH64:#define __FLT16_MIN_EXP__ (-13)
 // AARCH64:#define __FLT16_MIN__ 6.103515625e-5F16
 // AARCH64:#define __FLT_DENORM_MIN__ 1.40129846e-45F
 // AARCH64:#define __FLT_DIG__ 6