From 0c68a668fa932bba86acef929c6b388bca239a4b Mon Sep 17 00:00:00 2001 From: Chandler Carruth Date: Sun, 9 Jan 2011 21:00:19 +0000 Subject: Add a note about a missed FP optimization. llvm-svn: 123126 --- llvm/lib/Target/README.txt | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) (limited to 'llvm/lib') diff --git a/llvm/lib/Target/README.txt b/llvm/lib/Target/README.txt index 9ecd2ffc199..a9afffd95ae 100644 --- a/llvm/lib/Target/README.txt +++ b/llvm/lib/Target/README.txt @@ -2249,3 +2249,27 @@ S is only 6 bytes, but each element is 8 byte-aligned. We generate a loop and a memset. //===---------------------------------------------------------------------===// + +clang -O3 currently compiles this code: + +extern const int magic; +double f() { return 0.0 * magic; } + +into + +@magic = external constant i32 + +define double @_Z1fv() nounwind readnone { +entry: + %tmp = load i32* @magic, align 4, !tbaa !0 + %conv = sitofp i32 %tmp to double + %mul = fmul double %conv, 0.000000e+00 + ret double %mul +} + +We should be able to fold away this fmul to a constant, there is no 32-bit +integer which after sitofp will generate a NaN, inf, or -0.0. We should fold +this whenever the floating point type has enough exponent bits to represent +the largest integer value as < inf. + +//===---------------------------------------------------------------------===// -- cgit v1.2.3