[LLVMdev] Let's get rid of neverHasSideEffects

[reed kotler]

I'd personally like to see a moratorium on changes to tablegen until 
someone writes a reference manual for it that covers the application 
specific backends for code generation.

On 08/21/2012 02:02 PM, Jakob Stoklund Olesen wrote:
> All,
>
> TableGen likes to infer the MCID::UnmodeledSideEffects flag from an instruction's pattern. When an instruction doesn't have a pattern, it is assumed to have side effects.
>
> It's possible to override this behavior by setting neverHasSideEffects = 1.
>
> It was originally the intention that most instructions have patterns, but that's not the way it worked out. It is often more convenient to use def : Pat<>, and sometimes custom instruction selection is required.
>
> As a result, many instructions are defined without a pattern, and we often forget to set neverHasSideEffects = 1.
>
> $ grep -c UnmodeledSideEffects lib/Target/*/*GenInstrInfo.inc
> lib/Target/ARM/ARMGenInstrInfo.inc:727
> lib/Target/X86/X86GenInstrInfo.inc:920
>
> I don't think more than half of those UnmodeledSideEffects flags should be there.
>
>
> I want to stop inferring instruction properties from patterns in TableGen. It has become very hard to read instruction definitions when some properties are inferred, but only half the time.
>
> We can still use any patterns to emit TableGen warnings. If an instruction has a sideeffecting pattern, but hasSideEffects isn't set, TableGen should complain.
>
> I can't just kill off the neverHasSideEffects flag, that could cause miscompilations by clearing the UnmodeledSideEffects bit on instructions that should have it.
>
> Any suggestions for a clean transition? I was thinking about first adding a -Winfer flag that warns whenever TableGen infers a property that wasn't already set in the .td file.
>
> /jakob
>
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