[LLVMdev] DataLayout: Module vs Target

[Eric Christopher]

On Tue, Mar 10, 2015 at 9:43 PM Mehdi Amini <mehdi.amini at apple.com> wrote:

> On Mar 10, 2015, at 8:50 PM, Eric Christopher <echristo at gmail.com> wrote:
>
>
>
> On Tue, Mar 10, 2015, 5:19 PM Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> Hi all,
>
> The Language Reference says:
>
> "The function of the data layout string may not be what you expect.
> Notably, this is not a specification from the frontend of what alignment
> the code generator should use.
>
> Instead, if specified, the target data layout is required to match what
> the ultimate *code generator* expects. This string is used by the
> mid-level optimizers to improve code, and this only works if it matches
> what the ultimate code generator uses. If you would like to generate IR
> that does not embed this target-specific detail into the IR, then you don’t
> have to specify the string. This will disable some optimizations that
> require precise layout information, but this also prevents those
> optimizations from introducing target specificity into the IR.”
>
> I understand from this that the DataLayout present in the Module *must*
> match the DataLayout that the Target will use.
>
> There is no runtime check to enforce this at that time.
>
> My impression is that I can set the DataLayout on the Module to be
> big-endian and run the optimizer and the X86 backend.
>
> The IR-level passes will use the DataLayout supplied with the Module and
> the backend code will silently use the TargetMachine one.
>
> Should we enforce that the DataLayout attached to the Module matches the
> one expected by the Target?
>
>
> Definitely. That said clang does this in asserts mode.
>
> Can you be more specific?
> From what I see, clang (for an IR file only of course) will match the
> triple, but not the DataLayout (or I haven’t found where):
>
>
Sure, sorry, was on my phone earlier:

    std::string DLDesc =

        AsmHelper.TM->getDataLayout()->getStringRepresentation();

    if (DLDesc != TDesc) {

      unsigned DiagID = Diags.getCustomDiagID(

          DiagnosticsEngine::Error, "backend data layout '%0' does not
match "

                                    "expected target description '%1'");

      Diags.Report(DiagID) << DLDesc << TDesc;

lib/CodeGen/BackendUtil.cpp:656.

-eric


>     if (TheModule->getTargetTriple() != TargetOpts.Triple) {
>       CI.getDiagnostics().Report(SourceLocation(),
>                                  diag::warn_fe_override_module)
>           << TargetOpts.Triple;
>       TheModule->setTargetTriple(TargetOpts.Triple);
>     }
>
>
> Mehdi
>
>
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