Re: [PATCH v2 10/10] perf dwarf-aux: support DW_OP_piece expressions

[Zecheng Li]

On Sat, Aug 30, 2025 at 3:34 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Mon, Aug 25, 2025 at 07:58:33PM +0000, Zecheng Li wrote:
> > Support variables split across multiple registers or stack locations by
> > handling DW_OP_piece in DWARF expressions. This enables correct matching
> > of such variables by iterating over all pieces in the expression.
>
> Can you show me a real world example of these variables?

An optimizing compiler can sometimes split a struct (mostly stack
variable) to multiple locations, or even optimized out. DW_OP_piece
describes the location of each part. For example,

DW_OP_reg2 RCX, DW_OP_piece 0x4, DW_OP_breg7 RSP+28, DW_OP_piece 0x4

The first part is in RCX, the second part is in the memory location
RSP+28, each of size 0x4 byte. Memory access on 0(RCX) corresponds to
case 2 and 28(RSP) corresponds to case 1.

> >
> > There are two cases for matching memory access on the target register:
> >
> > 1. Accessing a struct member:
> >    - The type is the original variable's type.
> >    - The offset is the sum of the piece's offset and the operand's
> >      offset.
> > 2. Dereferencing a member:
> >    - The type is the member of the original variable (the member must be
> >      a pointer).
> >    - The size must match the piece size.
> >    - The access offset is the operand's offset.
> >
> > This change improves support for piece-wise variable locations in DWARF
> > expressions.