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.

llvm-svn: 128545

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp
index 964b04d..dea4a65 100644
--- a/llvm/unittests/ADT/APFloatTest.cpp
+++ b/llvm/unittests/ADT/APFloatTest.cpp
@@ -576,4 +576,27 @@ TEST(APFloatTest, StringHexadecimalExponentDeath) {
 #endif
 #endif
 
+TEST(APFloatTest, exactInverse) {
+  APFloat inv(0.0f);
+
+  // Trivial operation.
+  EXPECT_TRUE(APFloat(2.0).getExactInverse(&inv));
+  EXPECT_TRUE(inv.bitwiseIsEqual(APFloat(0.5)));
+  EXPECT_TRUE(APFloat(2.0f).getExactInverse(&inv));
+  EXPECT_TRUE(inv.bitwiseIsEqual(APFloat(0.5f)));
+
+  // FLT_MIN
+  EXPECT_TRUE(APFloat(1.17549435e-38f).getExactInverse(&inv));
+  EXPECT_TRUE(inv.bitwiseIsEqual(APFloat(8.5070592e+37f)));
+
+  // Large float
+  EXPECT_TRUE(APFloat(1.7014118e38f).getExactInverse(&inv));
+  EXPECT_TRUE(inv.bitwiseIsEqual(APFloat(5.8774718e-39f)));
+
+  // Zero
+  EXPECT_FALSE(APFloat(0.0).getExactInverse(0));
+  // Denormalized float
+  EXPECT_FALSE(APFloat(1.40129846e-45f).getExactInverse(0));
+}
+
 }