https://bugs.llvm.org/show_bug.cgi?id=46406
Bug ID: 46406
Summary: [X86] rsqrt and div generated for 1/sqrt(x * x * x)
with -Ofast
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: craig.top...@gmail.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
InstCombine transforms sqrt(x * x * x) into fabs(x) * sqrt(x) under fast math.
This means (1 / sqrt(x*x*x)) becomes (1 / (fabs(x) * sqrt(x))) which the
backend is unable to match to reciprocal sqrt approximation. Instead we do a
square root approximation which uses rsqrt followed by a regular divide.
Ideally we'd just do the x*x*x and then approximate the reciprocal square root
of that.
https://godbolt.org/z/fR66c4
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