[llvm-dev] Getting TargetLowering in AsmPrinter / Lowering constant addrspacecast

[Manuel Jacob via llvm-dev]

On 2015-12-15 00:59, Tom Stellard wrote:
> On Tue, Dec 15, 2015 at 12:41:57AM +0100, Manuel Jacob via llvm-dev 
> wrote:
>> Hi,
>> 
>> I'd like to add support for addrspacecast in initializers of global
>> variables, at least for the trivial case.  The trivial case is if
>> TargetLowering::isNoopAddrSpaceCast(SrcAS, DestAS) returns true.  In
>> this case the MCExpr for the addrspacecast is the MCExpr of its first
>> operand.
>> 
> 
> You could create a Util directory and put the implementation of
> isNoopAddrSpaceCast in there.  We do this in AMDGPU when we want
> to share code between the MC and non-MC parts of the compiler.

Your suggestion is very helpful to avoid code duplication for a single 
target.  However, it still requires every target to implement an 
additional target hook, instead of using the existing one.

-Manuel

> 
> -Tom
> 
>> It seems hard to obtain an instance of TargetLowering in
>> AsmPrinter::lowerConstant().  Other methods in this class obtain it by
>> calling `MF->getSubtarget().getTargetLowering()`.  However, because MF
>> is nullptr when lowering initializers of global variables, it won't
>> work.  My impression is that this is not possible because every 
>> function
>> has its own TargetLowering instance.  Is that correct?
>> 
>> NVPTX solves the problem by having a method on it's NVPTXAsmPrinter
>> class, which is mostly a copy of AsmPrinter::lowerConstant() and knows
>> how to handle addrspacecasts.  However, this probably isn't a solution
>> for e.g. X86.
>> 
>> Suggestions are highly welcome.
>> 
>> -Manuel
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