https://llvm.org/bugs/show_bug.cgi?id=25952
Bug ID: 25952
Summary: unreachable reduces performance
Product: new-bugs
Version: 3.7
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: g.bergmann.offic...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 15526
--> https://llvm.org/bugs/attachment.cgi?id=15526&action=edit
brainfuck interpreter and example data
Delaring cases in a switch statement like below as unreachable may reduce
performance:
switch x {
case 0: // ...
break;
case 1: // ...
break;
// ....
case 7: // ...
break;
default:
__builtin_unreachable();
}
Removing the __builtin_unreachable() improves the performance. On my PC the
attached example needs about 25 with it and 17 without.
The attached code is a simple brainfuck interpreter that can be tested by
calling "brainfuck-optimized mandel.brainfuck".
It was test with options -O3 and -O2 as 64 bit build on a haswell processor.
The problem is reproducible in Rust.
Profiling shows a suspiciously high number of branch mispredictions. From what
I have seen of the assembler code, I think that nested conditional branches
like a binary search are faster than a branch table by causing fewer
mispredictions.
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