[avr-llvm-devel] Fwd: [LLVMdev] A new builtin: __builtin_stack_pointer()

[Rick M. <rm...@la...>]

Just saw this on the LLVM list, is it in our AVR port as well?

Begin forwarded message:

> From: Behan Webster <be...@co...>
> Subject: [LLVMdev] A new builtin: __builtin_stack_pointer()
> Date: October 10, 2013 12:32:42 PDT
> To: "ll...@cs... List" <ll...@cs...>
> Cc: Mark Charlebois <cha...@gm...>
> 
> One of the issues the LLVMLinux project is having is with the use of
> named registers in the Linux kernel code. The kernel uses something like
> this in order to assign a C variable name to a register (one for each
> kernel arch).
> 
>    register unsigned long current_stack_pointer asm("esp");
> 
> clang doesn't allow this kind of thing which required a patch which less
> efficient:
> 
> #define current_stack_pointer ({ \
>       unsigned long esp; \
>       asm("mov %%esp, %0" : "=r"(esp)); \
>       esp; \
> })
> 
> This works for both gcc and clang, but often adds in 3 extra
> instructions since you need to copy the stack pointer to another
> register, but first that register needs to be saved to the stack and
> then restored after the stackpointer has been used; inefficient.
> 
> Another way would be to introduce a new builtin in parallel to others
> like __builtin_return_address(). Essentially adding a
> __builtin_stack_pointer() which does what the above code does. The idea
> would be to get something like this added to both clang and gcc in order
> to make this work across compilers, and across arches.
> 
> It ends up being a trivial patch for clang (see below). We're still
> looking for someone to help us on the gcc side.
> 
> The goal is to ideally make the kernel code work equally well with both
> compilers (clang and gcc).
> 
> Thoughts?
> 
> Thanks to Mark Charlebois for writing the patch below.
> 
> Behan
> 
> -- 
> Behan Webster
> be...@co...
> 
> 
> diff --git a/include/clang/Basic/Builtins.def b/include/clang/Basic/Builtins.def
> index 0a513ef..ca68f7e 100644
> --- a/include/clang/Basic/Builtins.def
> +++ b/include/clang/Basic/Builtins.def
> @@ -437,6 +437,7 @@ BUILTIN(__builtin_strstr, "c*cC*cC*", "nF")
> BUILTIN(__builtin_return_address, "v*IUi", "n")
> BUILTIN(__builtin_extract_return_addr, "v*v*", "n")
> BUILTIN(__builtin_frame_address, "v*IUi", "n")
> +BUILTIN(__builtin_stack_pointer, "v*", "n")
> BUILTIN(__builtin_flt_rounds, "i", "nc")
> BUILTIN(__builtin_setjmp, "iv**", "j")
> BUILTIN(__builtin_longjmp, "vv**i", "r")
> diff --git a/lib/CodeGen/CGBuiltin.cpp b/lib/CodeGen/CGBuiltin.cpp
> index d187678..f66f506 100644
> --- a/lib/CodeGen/CGBuiltin.cpp
> +++ b/lib/CodeGen/CGBuiltin.cpp
> @@ -736,6 +736,11 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
>     Value *F = CGM.getIntrinsic(Intrinsic::frameaddress);
>     return RValue::get(Builder.CreateCall(F, Depth));
>   }
> +  case Builtin::BI__builtin_stack_pointer: {
> +    Value *StackAddr =
> +      Builder.CreateCall(CGM.getIntrinsic(Intrinsic::stacksave));
> +    return RValue::get(StackAddr);
> +  }
>   case Builtin::BI__builtin_extract_return_addr: {
>     Value *Address = EmitScalarExpr(E->getArg(0));
>     Value *Result = getTargetHooks().decodeReturnAddress(*this, Address);
> 
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-- 
Rick