[LLVMdev] signext on function parameters and return.

[Daniel Sanders]

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Steve King
> Sent: 23 July 2015 01:45
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] signext on function parameters and return.
> 
> Hello,
> For a simple function taking a short and returning a short, clang
> generates IR with this function signature:
> 
> define signext i16 @foo(i16 signext %x)
>
> Some questions please:
> 
> 1) For the input parameter and return value, does the target control
> whether clang uses signext vs something else?  If so, how does this
> target query work?

Yes, the function signature in the IR is target specific since it already contains some ABI information at this point. I know Mips would use the definition above but other targets may vary.

The target hook is classifyArgumentType() and classifyReturnType() in tools/clang/lib/CodeGen/TargetInfo.cpp and returning ABIArgInfo::getExtend() causes the signext/zeroexts to be emitted appropriately for the type.

> 2) Does the presence of the signext mean it's imperative to sign
> extend, or that extension, only if needed, should be signed?

It's an agreement between the caller and callee to ensure the value is sign-extended. The exact meaning of this is target specific but for Mips it means that the values are sign-extended to full register width. Other targets might sign-extend to the nearest legal type.

So, for Mips at least, signext on a return causes the caller to assume the value is sign-extended and causes the callee to sign-extend the value. On an argument, it's the opposite way around, the caller sign-extends the value and the callee assumes it's sign extended.

> Thanks,
> -steve
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