https://bugs.llvm.org/show_bug.cgi?id=46115
Bug ID: 46115
Summary: -instcombine with fast flag moves div outside of loop
into loop, dramatically regressing performance
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: elr...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
Godbolt:
https://godbolt.org/z/3vuZbN
The code divides one vector by a scalar.
The unoptimized IR has
inverse = fdiv 1, scalar
and then multiplies one vector by the inverse in the loop.
With -instcombine, the division is moved into the loop.
Seems like a failure of cost modeling not to realize that divisions are far
more expensive.
With vectors of length 128, this leads to about a 5.6-fold regression in
performance on my 10980xe.
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