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Avoid turning a floating point division with a constant power of two into a denormal multiplication.

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Some platforms may treat denormals as zero, on other platforms multiplication
with a subnormal is slower than dividing by a normal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128555 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2011-03-30 17:02:54 +00:00
parent 546739656e
commit 8398512f89
3 changed files with 11 additions and 5 deletions
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8
lib/Support/APFloat.cpp
View File

@ -3583,6 +3583,14 @@ bool APFloat::getExactInverse(APFloat *inv) const {
if (reciprocal.divide(*this, rmNearestTiesToEven) != opOK)
return false;
// Avoid multiplication with a denormal, it is not safe on all platforms and
// may be slower than a normal division.
if (reciprocal.significandMSB() + 1 < reciprocal.semantics->precision)
return false;
assert(reciprocal.category == fcNormal &&
reciprocal.significandLSB() == reciprocal.semantics->precision - 1);
if (inv)
*inv = reciprocal;

2
test/Transforms/InstCombine/fdiv.ll
View File

@ -13,7 +13,7 @@ define float @test2(float %x) nounwind readnone ssp {
ret float %div
; CHECK: @test2
; CHECK-NEXT: fmul float %x, 0x3800000000000000
; CHECK-NEXT: fdiv float %x, 0x47E0000000000000
}
define float @test3(float %x) nounwind readnone ssp {

6
unittests/ADT/APFloatTest.cpp
View File

@ -589,10 +589,8 @@ TEST(APFloatTest, exactInverse) {
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)));
// Large float, inverse is a denormal.
EXPECT_FALSE(APFloat(1.7014118e38f).getExactInverse(0));
// Zero
EXPECT_FALSE(APFloat(0.0).getExactInverse(0));
// Denormalized float