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

[Justin Holewinski via llvm-dev]

Yeah, NVPTX is funny and requires some extra handling; especially in regards to how such casts are emitted in the assembly.  It’s certainly not ideal.  We actually wrap such casts in a custom MCExpr sub-class.




On 12/14/15, 6:41 PM, "Manuel Jacob" <me at manueljacob.de> 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.
>
>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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