[libc-commits] [PATCH] D154909: [libc] adjust strtofloat precision for subnormals

[Michael Jones via Phabricator via libc-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG2cb4731902d1: [libc] adjust strtofloat precision for subnormals (authored by michaelrj).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154909/new/

https://reviews.llvm.org/D154909

Files:
  libc/fuzzing/stdlib/strtofloat_fuzz.cpp


Index: libc/fuzzing/stdlib/strtofloat_fuzz.cpp
===================================================================
--- libc/fuzzing/stdlib/strtofloat_fuzz.cpp
+++ libc/fuzzing/stdlib/strtofloat_fuzz.cpp
@@ -24,6 +24,25 @@
 
 using __llvm_libc::fputil::FloatProperties;
 
+// This function calculates the effective precision for a given float type and
+// exponent. Subnormals have a lower effective precision since they don't
+// necessarily use all of the bits of the mantissa.
+template <typename F> inline int effective_precision(int exponent) {
+  int full_precision = FloatProperties<F>::MANTISSA_PRECISION;
+
+  // This is intended to be 0 when the exponent is the lowest normal and
+  // increase as the exponent's magnitude increases.
+  int bits_below_normal = (-exponent) - (FloatProperties<F>::EXPONENT_BIAS - 1);
+
+  // This comparison is optimized out by the compiler.
+  if (bits_below_normal >= 0 && bits_below_normal < full_precision - 1) {
+    // The precision should be the normal, full precision, minus the bits lost
+    // by this being a subnormal, minus one for the implicit leading one.
+    return full_precision - bits_below_normal - 1;
+  }
+  return full_precision;
+}
+
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   uint8_t *container = new uint8_t[size + 1];
   if (!container)
@@ -46,7 +65,7 @@
 
   size_t base = 0;
 
-  // This is just used to determine the base.
+  // This is just used to determine the base and precision.
   mpfr_t result;
   mpfr_init2(result, 256);
   mpfr_t bin_result;
@@ -69,6 +88,8 @@
     base = 10;
   }
 
+  auto result_exp = mpfr_get_exp(result);
+
   mpfr_clear(result);
   mpfr_clear(bin_result);
 
@@ -79,15 +100,15 @@
   // correct double is instead 66336650, which when converted to float is
   // rounded down to 66336648. This means we have to compare against the correct
   // precision to get the correct result.
+
   mpfr_t mpfr_float;
-  mpfr_init2(mpfr_float, FloatProperties<float>::MANTISSA_PRECISION);
+  mpfr_init2(mpfr_float, effective_precision<float>(result_exp));
 
   mpfr_t mpfr_double;
-  mpfr_init2(mpfr_double, FloatProperties<double>::MANTISSA_PRECISION);
+  mpfr_init2(mpfr_double, effective_precision<double>(result_exp));
 
   mpfr_t mpfr_long_double;
-  mpfr_init2(mpfr_long_double,
-             FloatProperties<long double>::MANTISSA_PRECISION);
+  mpfr_init2(mpfr_long_double, effective_precision<long double>(result_exp));
 
   // TODO: Add support for other rounding modes.
   mpfr_strtofr(mpfr_float, str_ptr, &out_ptr, base, MPFR_RNDN);


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