[LLVMdev] Enable changing UnsafeFPMath on a per-function basis

[Akira Hatanaka]

Basically my plan is to change the backend to use the value of
"unsafe-fp-math" that is recorded in the IR as a function attribute instead
of reading the value from TargetOptions::UnsafeFPMath (which is global). It
doesn't change at which level (function or instruction level) fast FP math
is tracked or make it harder to change selection sag to track it at a finer
granularity, if someone decides to do that later.

On Mon, Jan 12, 2015 at 12:54 PM, Reid Kleckner <rnk at google.com> wrote:

> Whatever happened to tracking the safe-or-fast-ness of FP math on
> instructions? Is tracking this property at a function granularity correct?
> I seem to recall nobody wanted to thread this through the SDAG.
>
> If we agree that we want to track this at the function level, then your
> proposal seems reasonable.
>
>
> On Thu, Jan 8, 2015 at 5:36 PM, Akira Hatanaka <ahatanak at gmail.com> wrote:
>
>> To continue the discussion I started last year (see the link below) on
>> embedding command-line options in bitcode, I came up with a plan to improve
>> the way the backend changes UnsafeFPMath on a per-function basis. The code
>> in trunk currently resets TargetOptions::UnsafeFPMath at the beginning of
>> SelectionDAGISel::runOnMachineFunction to enable compiling one function
>> with “unsafe-fp-math=true” and another with “unsafe-fp-math=false”, but
>> this won’t work if we want to parallelize the backend in the future, which
>> is something Eric mentioned in his talk last year.
>>
>> Here is the link to the original discussion I started last year:
>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-November/078785.html
>>
>> These are the changes I plan to make:
>>
>> 1. In llc.cpp (and any other tools that use the option), change the
>> function attribute in the IR based on the value of command line option
>> “enable-unsafe-pf-math” (EnableUnsafeFPMath, defined in CommandFlags.h).
>>
>> 2. Replace usages of TargetOptions::UnsafeFPMath with calls to a function
>> which gets the value of attribute “unsafe-fp-math” in the IR.
>>
>> 3. Stringify function attribute “unsafe-fp-math” and append it to the
>> string that is used as the lookup key in
>> TargetMachine::getSubtargetImpl(const Function&). Also, pass the function
>> attribute to the subtarget constructors that need it to construct itself
>> (e.g., ARM) or construct one of the backend objects (e.g., X86, which needs
>> it in the constructor of X86TargetLowering).
>>
>> 4. In MachineFunction’s constructor, call
>> TargetMachine::getSubtargetImpl(const Function &) to initialize STI
>> (pointer to the per-function subtarget object). Currently, the version of
>> TargetMachine::getSubtargetImpl that doesn’t take a const Function&
>> parameter is called, which returns the module-level subtarget object.
>>
>> 5.  Fix ARMAsmPrinter::emitAttributes to compute the value of
>> TargetOptions::UnsafeFPMath based on the function attributes of all the
>> functions in the module being compiled (see the link below).
>>
>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-December/079904.html
>>
>> 6. Move the code in CGCall.cpp that sets the function attributes to
>> BackendUtil.cpp. clang should set the function attributes regardless of
>> whether it is compiling from source code or from an IR file (e.g., clang
>> foo1.ll -o foo1.s -ffast-math), but currently this happens only if it’s
>> compiling from source.
>>
>> Any comments and suggestions are welcome. If we can agree on the main
>> issues, I'll follow up with patches that implement the changes I proposed.
>>
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>>
>
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