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/92175

From-SVN: r257415

diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index 91bebe5d232bb74305baeb7e52310eec404e6751..8c49b2b12da37b23d2934adce35ef1ccf315267b 100644 (file)
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-1927b40e59e7c2067ecb03384b331d1be3cb5eea
+02f11a2d5cf0db2c2675c13d92bb69529f2175dd
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

diff --git a/gcc/go/gofrontend/expressions.cc b/gcc/go/gofrontend/expressions.cc
index 8555c052980812559fc21c62eb36150ce555f6a0..f50de0a500448e4d95d2d22013193179cb010521 100644 (file)
--- a/gcc/go/gofrontend/expressions.cc
+++ b/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_neg_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 +16181,60 @@ 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_neg_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_neg_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 a negative zero.
+// BITS may be zero in which case it is ignored.
+
+bool
+Numeric_constant::is_float_neg_zero(const mpfr_t val, int bits)
+{
+  if (!mpfr_signbit(val))
+    return false;
+  if (mpfr_zero_p(val))
+    return true;
+  mp_exp_t min_exp;
+  switch (bits)
+    {
+    case 0:
+      return false;
+    case 32:
+      // In a denormalized float32 the exponent is -126, and there are
+      // 24 bits of which at least the last must be 1, so the smallest
+      // representable non-zero exponent is -126 - (24 - 1) == -149.
+      min_exp = -149;
+      break;
+    case 64:
+      // Minimum exponent is -1022, there are 53 bits.
+      min_exp = -1074;
+      break;
+    default:
+      go_unreachable();
+    }
+  return mpfr_get_exp(val) < min_exp;
 }
 
 // Get an int value.

diff --git a/gcc/go/gofrontend/expressions.h b/gcc/go/gofrontend/expressions.h
index d16a284b6b04c766430f7b0c6ee3c4ad11b0480c..a58c79c7b1512c7f99e47bbe3cb6831b18227159 100644 (file)
--- a/gcc/go/gofrontend/expressions.h
+++ b/gcc/go/gofrontend/expressions.h
@@ -4220,6 +4220,9 @@ class Numeric_constant
   bool
   check_complex_type(Complex_type*, bool, Location);
 
+  static bool
+  is_float_neg_zero(const mpfr_t, int bits);
+
   // The kinds of constants.
   enum Classification
   {