Re: Register encoding in assembly for load/store instructions

[Yonghong Song <yonghong.song@linux.dev>]

On 7/25/23 10:29 AM, Jose E. Marchesi wrote:
> 
> Hello Yonghong.
> 
> We have noticed that the llvm disassembler uses different notations for
> registers in load and store instructions, depending somehow on the width
> of the data being loaded or stored.
> 
> For example, this is an excerpt from the assembler-disassembler.s test
> file in llvm:
> 
>    // Note: For the group below w1 is used as a destination for sizes u8, u16, u32.
>    //       This is disassembler quirk, but is technically not wrong, as there are
>    //       no different encodings for 'r1 = load' vs 'w1 = load'.
>    //
>    // CHECK: 71 21 2a 00 00 00 00 00	w1 = *(u8 *)(r2 + 0x2a)
>    // CHECK: 69 21 2a 00 00 00 00 00	w1 = *(u16 *)(r2 + 0x2a)
>    // CHECK: 61 21 2a 00 00 00 00 00	w1 = *(u32 *)(r2 + 0x2a)
>    // CHECK: 79 21 2a 00 00 00 00 00	r1 = *(u64 *)(r2 + 0x2a)
>    r1 = *(u8*)(r2 + 42)
>    r1 = *(u16*)(r2 + 42)
>    r1 = *(u32*)(r2 + 42)
>    r1 = *(u64*)(r2 + 42)
> 
> The comment there clarifies that the usage of wN instead of rN in the
> u8, u16 and u32 cases is a "disassembler quirk".
> 
> Anyway, the problem is that it seems that `clang -S' actually emits
> these forms with wN.
> 
> Is that intended?

Yes, this is intended since alu32 mode is enabled where
w* registers are used for 8/16/32 bit load.

Note that for newer sign-extended loads, even at alu32 mode,
only r* register is used since the sign-extension extends
upto 64 bits for all variants (8/16/32).



>