Add APFloat::getExactInverse.
The idea is, that if an ieee 754 float is divided by a power of two, we can
turn the division into a cheaper multiplication. This function sees if we can
get an exact multiplicative inverse for a divisor and returns it if possible.
This is the hard part of PR9587.
I tested many inputs against llvm-gcc's frotend implementation of this
optimization and didn't find any difference. However, floating point is the
land of weird edge cases, so any review would be appreciated.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128545 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index 93806fa..abe5575 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -3562,3 +3562,29 @@
for (; I != NDigits; ++I)
Str.push_back(buffer[NDigits-I-1]);
}
+
+bool APFloat::getExactInverse(APFloat *inv) const {
+ // We can only guarantee the existance of an exact inverse for IEEE floats.
+ if (semantics != &IEEEhalf && semantics != &IEEEsingle &&
+ semantics != &IEEEdouble && semantics != &IEEEquad)
+ return false;
+
+ // Special floats and denormals have no exact inverse.
+ if (category != fcNormal)
+ return false;
+
+ // Check that the number is a power of two by making sure that only the
+ // integer bit is set in the significand.
+ if (significandLSB() != semantics->precision - 1)
+ return false;
+
+ // Get the inverse.
+ APFloat reciprocal(*semantics, 1ULL);
+ if (reciprocal.divide(*this, rmNearestTiesToEven) != opOK)
+ return false;
+
+ if (inv)
+ *inv = reciprocal;
+
+ return true;
+}