[PATCH v2] [compiler-rt] Avoid undefined behaviour in __floatsisf and __floatsidf

[Matthew Fernandez via llvm-commits]

These two functions for soft floating point support negate their signed int argument if it is 
negative. In the case where the argument is INT_MIN, this negation is undefined behaviour with 
respect to the C standard. This change performs the negation on an unsigned value, avoiding the 
just-described situation. This change does not alter the intended semantics of these functions.

Signed-off-by: Matthew Fernandez <matthew.fernandez at gmail.com>

Index: lib/builtins/floatsidf.c
===================================================================
--- lib/builtins/floatsidf.c	(revision 251670)
+++ lib/builtins/floatsidf.c	(working copy)
@@ -31,13 +31,14 @@

      // All other cases begin by extracting the sign and absolute value of a
      rep_t sign = 0;
+    unsigned int aAbs = a;
      if (a < 0) {
          sign = signBit;
-        a = -a;
+        aAbs = -aAbs;
      }

      // Exponent of (fp_t)a is the width of abs(a).
-    const int exponent = (aWidth - 1) - __builtin_clz(a);
+    const int exponent = (aWidth - 1) - __builtin_clz(aAbs);
      rep_t result;

      // Shift a into the significand field and clear the implicit bit.  Extra
@@ -44,7 +45,7 @@
      // cast to unsigned int is necessary to get the correct behavior for
      // the input INT_MIN.
      const int shift = significandBits - exponent;
-    result = (rep_t)(unsigned int)a << shift ^ implicitBit;
+    result = (rep_t)aAbs << shift ^ implicitBit;

      // Insert the exponent
      result += (rep_t)(exponent + exponentBias) << significandBits;
Index: lib/builtins/floatsisf.c
===================================================================
--- lib/builtins/floatsisf.c	(revision 251670)
+++ lib/builtins/floatsisf.c	(working copy)
@@ -31,23 +31,24 @@

      // All other cases begin by extracting the sign and absolute value of a
      rep_t sign = 0;
+    unsigned int aAbs = a;
      if (a < 0) {
          sign = signBit;
-        a = -a;
+        aAbs = -aAbs;
      }

      // Exponent of (fp_t)a is the width of abs(a).
-    const int exponent = (aWidth - 1) - __builtin_clz(a);
+    const int exponent = (aWidth - 1) - __builtin_clz(aAbs);
      rep_t result;

      // Shift a into the significand field, rounding if it is a right-shift
      if (exponent <= significandBits) {
          const int shift = significandBits - exponent;
-        result = (rep_t)a << shift ^ implicitBit;
+        result = (rep_t)aAbs << shift ^ implicitBit;
      } else {
          const int shift = exponent - significandBits;
-        result = (rep_t)a >> shift ^ implicitBit;
-        rep_t round = (rep_t)a << (typeWidth - shift);
+        result = (rep_t)aAbs >> shift ^ implicitBit;
+        rep_t round = (rep_t)aAbs << (typeWidth - shift);
          if (round > signBit) result++;
          if (round == signBit) result += result & 1;
      }