[llvm-dev] RFC: callee saved register verifier

Sanjoy Das via llvm-dev llvm-dev at lists.llvm.org
Fri May 13 16:04:30 PDT 2016

Hi Quentin,

Thanks for the quick response!

Quentin Colombet wrote:
 > You mean with code generated by another compiler/handwritten,
 > because otherwise, this is pretty easy to verify in the machine
 > verifier :).
 > (We probably do not verify that though, but we should.)
 > Thus, could you elaborate on the use cases.

The runtime functions in question are usually hand-written assembly,
or generated from another ("tier-1") JIT.

I expect this to be more useful when calling into hand-written
assembly routines than when calling into other-compiler-generated
code, since hand-written assembly tends to be more prone to the kind
of mistakes I'm verifying against (accidentally clobbering a CSR on a
rarely taken path).

So far we've not seen any bugs around _LLVM_ failing to obey a calling
convention, so (as yet! :) ) I'm not too worried around LLVM
miscompiling due to some exotic calling convention.

-- Sanjoy

 >> Implementation wise, I have a rough prototype that works by running a
 >> MachineFunctionPass before register allocation that emits the
 >> necessary MachineInstrs / control flow to make this happen.  Is that a
 >> right way to solve the problem?
 > I’d say yes, assuming the use cases do not involve llvm, otherwise 
the verifier approach seems preferable to me.
 >> If so, I'll clean up what I have and
 >> put it up for review on llvm-commits.  If there are better ways to do
 >> this, then I'm happy to hear them.
 >> -- Sanjoy
 > Cheers,
 > -Quentin

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