https://bugs.llvm.org/show_bug.cgi?id=50412
Bug ID: 50412
Summary: Excessive unrolling/vectorization and ignored
assumption for factorial
Product: tools
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: opt
Assignee: unassignedb...@nondot.org
Reporter: l...@rifkin.dev
CC: llvm-bugs@lists.llvm.org
LLVM is generating suboptimal code for a simple factorial function due to
excessive unrolling/vectorization. https://godbolt.org/z/fPazKsEsn.
Three levels of unrolling/vectorization seem to be generated: n >= 32, 32 >= n
>= 8, and n < 8.
Because this function is working with 32-bit ints, anything larger than 12!
isn't particularly useful (20! for 64-bit ints). The n >= 32 level should never
be used.
It would be cool if the compiler could deduce this, but perhaps not possible.
Even with explicitly telling the compiler the domain of the function, clang
still generates the same excessive code.
int f(int n) {
__builtin_assume(n <= 12);
if(n <= 0) return 1;
return n * f(n-1);
}
The n >= 32 level should not be generated given the assumption.
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