compiler: avoid negative zero in float constants

    
    Check for negative numbers with very small magnitudes that will round
    to negative zero, and force them to positive zero instead.
    
    This implements the spec clarification in https://golang.org/cl/14727.
    The test is in https://golang.org/cl/91895.
    
    Fixes golang/go#12621
    
    Reviewed-on: https://go-review.googlesource.com/91915

From-SVN: r257390

gcc/go/gofrontend/MERGE

-36594b69b94326014c331fe50a5a345ef4f8de16
+7eebd495df915ab87926b8dd88f554674cfdacea
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -16158,10 +16158,16 @@ Numeric_constant::set_float(Type* type, const mpfr_t val)
   this->clear();
   this->classification_ = NC_FLOAT;
   this->type_ = type;
+
   // Numeric constants do not have negative zero values, so remove
   // them here.  They also don't have infinity or NaN values, but we
   // should never see them here.
-  if (mpfr_zero_p(val))
+  int bits = 0;
+  if (type != NULL
+      && type->float_type() != NULL
+      && !type->float_type()->is_abstract())
+    bits = type->float_type()->bits();
+  if (Numeric_constant::is_float_zero(val, bits))
     mpfr_init_set_ui(this->u_.float_val, 0, GMP_RNDN);
   else
     mpfr_init_set(this->u_.float_val, val, GMP_RNDN);
@@ -16175,8 +16181,50 @@ Numeric_constant::set_complex(Type* type, const mpc_t val)
   this->clear();
   this->classification_ = NC_COMPLEX;
   this->type_ = type;
+
+  // Avoid negative zero as in set_float.
+  int bits = 0;
+  if (type != NULL
+      && type->complex_type() != NULL
+      && !type->complex_type()->is_abstract())
+    bits = type->complex_type()->bits() / 2;
+
+  mpfr_t real;
+  mpfr_init_set(real, mpc_realref(val), GMP_RNDN);
+  if (Numeric_constant::is_float_zero(real, bits))
+    mpfr_set_ui(real, 0, GMP_RNDN);
+
+  mpfr_t imag;
+  mpfr_init_set(imag, mpc_imagref(val), GMP_RNDN);
+  if (Numeric_constant::is_float_zero(imag, bits))
+    mpfr_set_ui(imag, 0, GMP_RNDN);
+
   mpc_init2(this->u_.complex_val, mpc_precision);
-  mpc_set(this->u_.complex_val, val, MPC_RNDNN);
+  mpc_set_fr_fr(this->u_.complex_val, real, imag, MPC_RNDNN);
+
+  mpfr_clear(real);
+  mpfr_clear(imag);
+}
+
+// Return whether VAL, at a precision of BITS, is zero.  BITS may be
+// zero in which case it is ignored.
+
+bool
+Numeric_constant::is_float_zero(const mpfr_t val, int bits)
+{
+  if (mpfr_zero_p(val))
+    return true;
+  switch (bits)
+    {
+    case 0:
+      return false;
+    case 32:
+      return mpfr_get_flt(val, GMP_RNDN) == 0;
+    case 64:
+      return mpfr_get_d(val, GMP_RNDN) == 0;
+    default:
+      go_unreachable();
+    }
 }
 
 // Get an int value.

gcc/go/gofrontend/expressions.h

@@ -4220,6 +4220,9 @@ class Numeric_constant
   bool
   check_complex_type(Complex_type*, bool, Location);
 
+  static bool
+  is_float_zero(const mpfr_t, int bits);
+
   // The kinds of constants.
   enum Classification
   {