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From: "llvm at rifkin dot dev" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101701] New: GCC optimization and code generation for
 if-else chains vs ternary chains vs a switch
Date: Fri, 30 Jul 2021 23:37:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101701

            Bug ID: 101701
           Summary: GCC optimization and code generation for if-else
                    chains vs ternary chains vs a switch
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

I'm looking at an example of three equivalent functions implemented with
if-else chains, ternary chains, and a switch. Gcc is not compiling them
equivalently: https://godbolt.org/z/8cjGr7M7W.

For the if-else chain, gcc does not optimize away the jumps.
For the ternary chain, gcc does its codegen well.
For the switch, gcc also does its codegen well but there is an extra mov
instruction compared to the ternary chain.

I don't think it's idealistic to say these should compile equivalently - if
someone told me to prefer one over the other for performance reasons I'd
dismiss it as a micro-optimization.

Clang does not do this perfectly either at the moment.




This bug is probably miscategorized. I am not sure the correct category for=
 it.=