[llvm] r203469 - Fix regression with -O0 for mips .

[Eric Christopher]

> Sorry for not explaining this in the commit message. There was a bug filed
> against this and it was more completely explained there.
>
> http://llvm.org/bugs/show_bug.cgi?id=19069
>
> The original problem that the optimize pass was created for is a problem
> that occurs with pic when lazy binding is involved.
> It shows up in an extremely acute form with mips16 hard float but can occur
> with Mips32 also.
>
> What happens is that if the first time something is called through the GOT,
> the compiler may save the address of the routine in the GOT ; but that
> routine is meant to only be called once and do the resolution and then
> replace itself with the address of the loaded routine. But the compiler was
> saving away the address of the fixup routine so then it keeps doing the
> fixup up many many times which can be in a loop.
> (This was also a gcc bug a long time ago when lazy binding was introduced
> for mips).
>

Yup. I recall.

> So the LLVM compiler was changed to optimize this but the change requires
> assumes that this pass is always being called now for one of the basic ABI
> requirements of Mips; which is that the pic call be of the form JALR $25.
>
> If this pass is not called, then any register can be used and that will
> break the ABI.
>
> Our pass was added to a the optimize SSA pass which the pass manager will
> not call if -O0 is specified.
>

Hrm. Would we be better off running the pass in the same place either
way? Also, there are no comments, this sounds like a great place for
some comments :) Otherwise someone could delete the line and wonder
what the heck.

> The next place in pass manager after the optimize SSA pass is register
> allocation; so by adding this to pre register allocation in the case of -O0,
> all is good again.
>

Probably should rename the pass then. It's no longer an optimize, it's
a type of CodeGenPrepare in my mind. Also, is preRA the best place?


> Without this; -O0 is broken.
> I've run all of test-suite at -O0 with the patch and it works fine and we
> are adding -O0 test-suite runs to our nightly build bots so we don't let
> something like this slip through again.
>

:)

-eric

> :)
>
>
> Reed
>>
>> On Mon, Mar 10, 2014 at 9:31 AM, Reed Kotler
>> <rkotler-8NJIiSa5LzA at public.gmane.org> wrote:
>>>
>>> Author: rkotler
>>> Date: Mon Mar 10 11:31:25 2014
>>> New Revision: 203469
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=203469&view=rev
>>> Log:
>>> Fix regression with -O0 for mips .
>>>
>>>
>>> Added:
>>>      llvm/trunk/test/CodeGen/Mips/optimize-pic-o0.ll
>>> Modified:
>>>      llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp
>>>
>>> Modified: llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp?rev=203469&r1=203468&r2=203469&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp (original)
>>> +++ llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp Mon Mar 10 11:31:25
>>> 2014
>>> @@ -175,6 +175,9 @@ public:
>>>     virtual bool addInstSelector();
>>>     virtual void addMachineSSAOptimization();
>>>     virtual bool addPreEmitPass();
>>> +
>>> +  virtual bool addPreRegAlloc();
>>> +
>>>   };
>>>   } // namespace
>>>
>>> @@ -208,6 +211,15 @@ void MipsPassConfig::addMachineSSAOptimi
>>>     TargetPassConfig::addMachineSSAOptimization();
>>>   }
>>>
>>> +bool MipsPassConfig::addPreRegAlloc() {
>>> +  if (getOptLevel() == CodeGenOpt::None) {
>>> +    addPass(createMipsOptimizePICCallPass(getMipsTargetMachine()));
>>> +    return true;
>>> +  }
>>> +  else
>>> +    return false;
>>> +}
>>> +
>>>   void MipsTargetMachine::addAnalysisPasses(PassManagerBase &PM) {
>>>     if (Subtarget.allowMixed16_32()) {
>>>       DEBUG(errs() << "No ");
>>>
>>> Added: llvm/trunk/test/CodeGen/Mips/optimize-pic-o0.ll
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/Mips/optimize-pic-o0.ll?rev=203469&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/CodeGen/Mips/optimize-pic-o0.ll (added)
>>> +++ llvm/trunk/test/CodeGen/Mips/optimize-pic-o0.ll Mon Mar 10 11:31:25
>>> 2014
>>> @@ -0,0 +1,33 @@
>>> +; RUN: llc -mtriple=mipsel -O0 < %s | FileCheck %s
>>> +
>>> +; Function Attrs: nounwind
>>> +define i32 @main()  {
>>> +entry:
>>> +  %retval = alloca i32, align 4
>>> +  %i = alloca i32, align 4
>>> +  store i32 0, i32* %retval
>>> +  store i32 0, i32* %i, align 4
>>> +  br label %for.cond
>>> +
>>> +for.cond:                                         ; preds = %for.inc,
>>> %entry
>>> +  %0 = load i32* %i, align 4
>>> +  %cmp = icmp slt i32 %0, 10
>>> +  br i1 %cmp, label %for.body, label %for.end
>>> +
>>> +for.body:                                         ; preds = %for.cond
>>> +  call void bitcast (void (...)* @foo to void ()*)()
>>> +; CHECK: jalr $25
>>> +  br label %for.inc
>>> +
>>> +for.inc:                                          ; preds = %for.body
>>> +  %1 = load i32* %i, align 4
>>> +  %inc = add nsw i32 %1, 1
>>> +  store i32 %inc, i32* %i, align 4
>>> +  br label %for.cond
>>> +
>>> +for.end:                                          ; preds = %for.cond
>>> +  %2 = load i32* %retval
>>> +  ret i32 %2
>>> +}
>>> +
>>> +declare void @foo(...)
>>>
>>>
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>
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