[LLVMdev] Backend : What am I missing here

[Jiong Wang]

Hi Lee,

on 2013/4/24 17:15, Lee Hammerton wrote:
> Hi,
>
> While working on adding newlib to my backend I hit the following assert :
>
> Assertion failed: BestRC && "Couldn't find the register class", file 
> TargetRegisterInfo.cpp, line 112
>
> The following llvm causes this :
>
> define void @specialBreak(i8* %out) nounwind {
> entry:
>   %out.addr = alloca i8*, align 4
>   store i8* %out, i8** %out.addr, align 4
>   %0 = load i8** %out.addr, align 4
>   %call = call signext i8 @cnv(i8* %0)
>   %1 = load i8** %out.addr, align 4
>   %arrayidx = getelementptr inbounds i8* %1, i32 0
>   store i8 %call, i8* %arrayidx, align 1
>   ret void
> }
>
> declare signext i8 @cnv(i8*)
>
> Now originally (at the time I hadn't added 8/16 bit register support) 
> I had made the calling convention return values promote i8 & i16 to i32 :
>
> def RetCC_test : CallingConv<[
>   // Promote i8 returns to i32.
>   CCIfType<[i8], CCPromoteToType<i32>>,
>   // Promote i16 returns to i32.
>   CCIfType<[i16], CCPromoteToType<i32>>,
>
>   // i32 are returned in register R0
>   CCIfType<[i32], CCAssignToReg<[R0L]>>
> ]>;

 >>> if you do not override getTypeForExtArgOrReturn,  i8, i16 will be 
promoted to i32 automatically.

>
> I've now changed this to :
>
> def RetCC_test : CallingConv<[
>   // i8 are returned in register R0
>   CCIfType<[i8], CCAssignToReg<[R0B]>>,
>   // i16 are returned in register R0
>   CCIfType<[i16], CCAssignToReg<[R0W]>>,
>   // i32 are returned in register R0
>   CCIfType<[i32], CCAssignToReg<[R0L]>>
> ]>;
>
> Which has stopped the assert and produced valid code.
>
> I`m happy to leave the return value definitions as they are now, but 
> was wondering if it perhaps points to something I've missed elsewhere 
> as to why the original promoted convention would fail.

  >>> I suggest you use gdb to see what's the physical reg num, and you 
can check the generated XXXGenRegisterInfo.inc, maybe it will help you 
to find out what you are missing.

>
>  Lee
>
>
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