diff options
| -rw-r--r-- | lib/Target/README.txt | 22 |
1 files changed, 19 insertions, 3 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 4032c333f7..dfa00b8c95 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -2109,7 +2109,7 @@ aggressively as malloc though. //===---------------------------------------------------------------------===// -clang -03 doesn't optimize this: +clang -O3 doesn't optimize this: void f1(int* begin, int* end) { std::fill(begin, end, 0); @@ -2253,12 +2253,28 @@ not an INF. The CannotBeNegativeZero predicate in value tracking should be extended to support general "fpclassify" operations that can return yes/no/unknown for each of these predicates. -In this predicate, we know that [us]itofp is trivially never NaN or -0.0, and +In this predicate, we know that uitofp is trivially never NaN or -0.0, and we know that it isn't +/-Inf if the floating point type has enough exponent bits to represent the largest integer value as < inf. //===---------------------------------------------------------------------===// +When optimizing a transformation that can change the sign of 0.0 (such as the +0.0*val -> 0.0 transformation above), it might be provable that the sign of the +expression doesn't matter. For example, by the above rules, we can't transform +fmul(sitofp(x), 0.0) into 0.0, because x might be -1 and the result of the +expression is defined to be -0.0. + +If we look at the uses of the fmul for example, we might be able to prove that +all uses don't care about the sign of zero. For example, if we have: + + fadd(fmul(sitofp(x), 0.0), 2.0) + +Since we know that x+2.0 doesn't care about the sign of any zeros in X, we can +transform the fmul to 0.0, and then the fadd to 2.0. + +//===---------------------------------------------------------------------===// + clang -O3 currently compiles this code: #include <emmintrin.h> @@ -2270,7 +2286,7 @@ into define i32 @_Z1fd(double %x) nounwind readnone { entry: %vecinit.i = insertelement <2 x double> undef, double %x, i32 0 - %vecinit1.i = insertelement <2 x double> %vecinit.i, double 0.000000e+00, i32 1 + %vecinit1.i = insertelement <2 x double> %vecinit.i, double 0.000000e+00,i32 1 %0 = tail call i32 @llvm.x86.sse2.cvtsd2si(<2 x double> %vecinit1.i) nounwind ret i32 %0 } |