llvm dev - Dec 2015 - Getting TargetLowering in AsmPrinter / Lowering constant addrspacecast

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

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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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.

-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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev