[llvm] r258847 - Allow X86::COND_NE_OR_P and X86::COND_NP_OR_E to be reversed.

[Benjamin Kramer via llvm-commits]

This also breaks compiling code with

>  fatal error: error in backend: Undefined temporary symbol

will revert and follow up with a test case.

On Wed, Jan 27, 2016 at 5:11 AM, James Knight via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> This change is broken in some fashion; it causes llvm to rewrite branches incorrectly producing an infinite loop.
>
>> On Jan 26, 2016, at 3:08 PM, Cong Hou via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>>
>> Author: conghou
>> Date: Tue Jan 26 14:08:01 2016
>> New Revision: 258847
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=258847&view=rev
>> Log:
>> Allow X86::COND_NE_OR_P and X86::COND_NP_OR_E to be reversed.
>>
>> Currently, AnalyzeBranch() fails non-equality comparison between floating points
>> on X86 (see https://llvm.org/bugs/show_bug.cgi?id=23875). This is because this
>> function can modify the branch by reversing the conditional jump and removing
>> unconditional jump if there is a proper fall-through. However, in the case of
>> non-equality comparison between floating points, this can turn the branch
>> "unanalyzable". Consider the following case:
>>
>> jne.BB1
>> jp.BB1
>> jmp.BB2
>> .BB1:
>> ...
>> .BB2:
>> ...
>>
>> AnalyzeBranch() will reverse "jp .BB1" to "jnp .BB2" and then "jmp .BB2" will be
>> removed:
>>
>> jne.BB1
>> jnp.BB2
>> .BB1:
>> ...
>> .BB2:
>> ...
>>
>> However, AnalyzeBranch() cannot analyze this branch anymore as there are two
>> conditional jumps with different targets. This may disable some optimizations
>> like block-placement: in this case the fall-through behavior is enforced even if
>> the fall-through block is very cold, which is suboptimal.
>>
>> Actually this optimization is also done in block-placement pass, which means we
>> can remove this optimization from AnalyzeBranch(). However, currently
>> X86::COND_NE_OR_P and X86::COND_NP_OR_E are not reversible: there is no defined
>> negation conditions for them.
>>
>> In order to reverse them, this patch defines two new CondCode X86::COND_E_AND_NP
>> and X86::COND_P_AND_NE. It also defines how to synthesize instructions for them.
>> Here only the second conditional jump is reversed. This is valid as we only need
>> them to do this "unconditional jump removal" optimization.
>>
>>
>> Differential Revision: http://reviews.llvm.org/D11393
>>
>>
>>
>> Modified:
>>    llvm/trunk/lib/Target/X86/X86InstrInfo.cpp
>>    llvm/trunk/lib/Target/X86/X86InstrInfo.h
>>    llvm/trunk/test/CodeGen/X86/block-placement.ll
>>    llvm/trunk/test/CodeGen/X86/fast-isel-cmp-branch2.ll
>>    llvm/trunk/test/CodeGen/X86/fast-isel-cmp-branch3.ll
>>    llvm/trunk/test/CodeGen/X86/fp-une-cmp.ll
>>
>
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