[llvm-dev] Strange Machineinstr

[Krzysztof Parzyszek via llvm-dev]

That's a store-multiple that saves a few registers at the entry to the 
function. It's a part of the frame setup and it's generated during 
prolog/epilog insertion.

The MIR code that you're showing is from before the frame creation, so 
it does not contain the instructions that do frame setup/cleanup.

$noreg means "no register". It's used when an instruction required an 
operand that is a register, but none is specified. It's like a 
null-pointer but for registers. The instructions represented by 
MachineInstr do not have to match hardware instructions directly, many 
of them are instructions for compiler's internal use, so you can see 
things that don't match the final assembly.

-Krzysztof


On 6/15/2018 11:28 AM, Muhui Jiang via llvm-dev wrote:
> Hi
> 
> I write a machinefunction pass  to print all the machinefunction's 
> machine instructions.
> 
> My target architecture is ARM. However, I don't understand some part of 
> the machine instructions.
> 
> Below is some of the assembly language for function A.
> 
>     .text:0001C034                 STMFD   SP!, {R4,R10,R11,LR}
>     .text:0001C038                 ADD     R11, SP, #8
>     .text:0001C03C                 SUB     SP, SP, #0x20
>     .text:0001C040                 STR     R0, [R11,#statbuf]
>     .text:0001C044                 STR     R1, [SP,#0x28+var_14]
>     .text:0001C048                 LDR     R0, [SP,#0x28+var_14]
>     .text:0001C04C                 LDR     R0, [R0]
>     .text:0001C050                 STR     R0, [SP,#0x28+ts]
>     .text:0001C054                 LDR     R0, [SP,#0x28+ts]
>     .text:0001C058                 LDR     R0, [R0,#4]
>     .text:0001C05C                 CMN     R0, #0xC0000002
>     .text:0001C060                 BNE     loc_1C088
>     .text:0001C064                 B       loc_1C068
> 
> 
> Below is the corresponding MachineInstr
> 
>         %1:gpr = COPY $r1
> 
>         %0:gpr = COPY $r0
> 
>         %3:gpr = COPY %1:gpr
> 
>         %2:gpr = COPY %0:gpr
> 
>         STRi12 %0:gpr, %stack.1.statbuf.addr, 0, 14, $noreg :: (store 4
>         into %ir.statbuf.addr)
> 
>         STRi12 %1:gpr, %stack.2.ts.addr, 0, 14, $noreg :: (store 4 into
>         %ir.ts.addr)
> 
>         %4:gpr = LDRi12 %stack.2.ts.addr, 0, 14, $noreg
> 
>         %5:gpr = LDRi12 killed %4:gpr, 0, 14, $noreg
> 
>         STRi12 killed %5:gpr, %stack.3.timespec, 0, 14, $noreg
> 
>         %6:gpr = LDRi12 %stack.3.timespec, 0, 14, $noreg
> 
>         %7:gpr = LDRi12 killed %6:gpr, 4, 14, $noreg
> 
>         CMNri killed %7:gpr, -1073741822, 14, $noreg
> 
>         Bcc %bb.3, 1, $cpsr
> 
>         B %bb.1
> 
> 
> I don't know how "STMFD   SP!, {R4,R10,R11,LR}" is translated into the 
> machineinstr. Also, what does $noreg mean? what does gpr and the value 
> in front of it mean? It would be great if someone who are familiar with 
> this can explain it to me or give me some reference. Thank you very much.
> 
> Regards
> Muhui
> 
> 
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