https://bugs.llvm.org/show_bug.cgi?id=49763
Bug ID: 49763
Summary: Sub-optimal optimization of abs(x) % n
Product: tools
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: opt
Assignee: unassignedb...@nondot.org
Reporter: rif...@purdue.edu
CC: llvm-bugs@lists.llvm.org
LLVM does not find all optimizations related to taking the remainder of the
absolute value of a signed number.
The following equivilent expressions result in sub-optimal code generation:
x % 2 == 1 || x % 2 == -1
std::abs(x % 2) == 1
std::abs(x) % 2 == 1
LLVM may be ignoring poison values for std::abs / @llvm.abs.* in making
optimizations for the remainder operation. Alternatively, LLVM may not be
taking
the absolute value into account when performing algebraic operations on the
remainder instructions.
If x is guaranteed to be positive (e.g. it is unsigned or an induction
variable),
LLVM does produce expected codegen. When std::abs is present, LLVM should be
able
to take advantage of the remainder operation being odd (i.e. -x % n = -(x %
n)).
The expected codegen is that of x % 2 != 0, x & 1, where LLVM generates a
single
and instruction.
Godbolt comparison of these expressions: https://godbolt.org/z/1YPW1z4v7
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