================ @@ -6178,10 +6178,16 @@ The current supported opcode vocabulary is limited: the last entry from the second last entry and appends the result to the expression stack. - ``DW_OP_plus_uconst, 93`` adds ``93`` to the working expression. -- ``DW_OP_LLVM_fragment, 16, 8`` specifies the offset and size (``16`` and ``8`` - here, respectively) of the variable fragment from the working expression. Note - that contrary to DW_OP_bit_piece, the offset is describing the location - within the described source variable. +- ``DW_OP_LLVM_fragment, 16, 8`` specifies that the top of the expression stack + is a fragment of the source language variable with the given offset and size + (``16`` and ``8`` here, respectively). Note that the offset and size are the + opposite way around to ``DW_OP_bit_piece``, and the offset is within the + source language variable. +- ``DW_OP_bit_piece, 8, 16`` specifies that the source language variable can be + found in the sequence of bits at the given size and offset (``8`` and ``16`` + here, respectively) within the top of the expression stack. Note that the + offset and size are the opposite way around to ``DW_OP_LLVM_fragment``, and the + offset is within the LLVM variable (if that's at the top of the stack). ---------------- john-brawn-arm wrote:
Ah, I see, I hadn't noticed that (I'd just read the description of DW_OP_bit_piece and hadn't read the general description of composite location descriptions). I had been understanding something like "DW_OP_reg0 RAX, DW_OP_bit_piece 0x3 0x0" to mean "this variable can be found in the 3 bits at offset 0 in RAX", but it actually means something closer to "the bottom 3 bits of this variable can be found in the 3 bits at offset 0 in RAX". I think this doesn't make much difference though if we restrict DW_OP_bit_piece in the llvm.dbg intrinsics to use it only for its ability to describe a value stored in part of a register, with the other bits not being described, as I think that's what the behaviour I've currently implemented is. Though maybe it would be better to instead use DW_OP_and and DW_OP_shr to extract the bits from the register. I'll think about this some more. https://github.com/llvm/llvm-project/pull/85665 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
