Teach fpcmp about the x. FP format

[rovka]

Summary

At the moment fpcmp rejects floating point numbers with no decimal
digits after the decimal point, such as "3.". However, flang-new often
prints floating point numbers in this format and it seems to be
a perfectly respectable format (strtod is happy enough to gobble it up).

This patch updates fpcmp to allow the absence of post-decimal digits
if pre-decimal digits are present.

[Meinersbur]

I fear there is the problem that any single dot will be interpreted as a number. I tried strtod(".") (https://godbolt.org/z/Ke9n4zbn5) and it doesn't gobble it up. So we need to check that there are either digits before or after the period, but cannot omit both.

I don't think we need the compatibility flag. This patch would make fpcmp only more tolerant, so there would be any new test failures.

[rovka]

Looks like flang isn't using this format anymore. We can resurrect this patch some other time if we have a need for it. Thanks for the comments though!

[rovka]

I think I got myself confused, flang does still print values in this format, and that's what's going to be in the reference outputs. Therefore, if we want to be able to pass this test when FCVS_ALLOW_FLEXIBLE_OUTPUT is enabled, we need to teach fpcmp how to handle this.

Updated the patch per @Meinersbur 's comments, thanks!

tools/fpcmp.c

static const char *AdvanceNumber(const char *StartPos, const char *End) {
   const char *Pos = StartPos;
   const char *EndOfNumber = NULL;
 
   // Sign character (optional)
   if (Pos < End && isSignedChar(*Pos))
     ++Pos;
 
   // Pre-decimal digits (optional)
+  bool HasPreDecimalDigit = false;
   while (Pos < End && isDigitChar(*Pos)) {
     ++Pos;
     EndOfNumber = Pos;
+    HasPreDecimalDigit = true;
   }
 
   // Decimal separator
   if (Pos < End && *Pos == '.') {
     ++Pos;
 
-    // Post-decimal digits (require at least one when period present)
+    // Only update the end of number if we're sure if we have a number (i.e. we
+    // have seen at least one digit so far).
+    if (HasPreDecimalDigit)
+      EndOfNumber = Pos;
+
+    // Post-decimal digits (optional if we have pre-decimal digits)
     bool HasPostDecimalDigit = false;
     while (Pos < End && isDigitChar(*Pos)) {
       HasPostDecimalDigit = true;
 
       ++Pos;
       EndOfNumber = Pos;
     }
-    if (!HasPostDecimalDigit)
+    if (!HasPreDecimalDigit && !HasPostDecimalDigit)
       return EndOfNumber;
   }
 
   // Require a valid number before the exponent.
   // (e.g. don't recognize '+e1' as a number)
   if (!EndOfNumber)
     return EndOfNumber;