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<p class="MsoNormal">A “location description [that] is a register operation” is the language in DWARF v3; in later versions, it is “a simple register location description.” This means something like DW_OP_reg5, which is allowed in a location description but
not in a DWARF expression.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Form DW_FORM_data4, value 0, would be interpreted as a location list reference. It is not by itself an address or a register location description.<o:p></o:p></p>
<p class="MsoNormal">--paulr<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="mso-ligatures:none">From:</span></b><span style="mso-ligatures:none"> Dwarf-discuss <dwarf-discuss-bounces+paul.robinson=sony.com@lists.dwarfstd.org>
<b>On Behalf Of </b>Vsevolod Alekseyev via Dwarf-discuss<br>
<b>Sent:</b> Monday, September 18, 2023 10:08 AM<br>
<b>To:</b> dwarf-discuss@lists.dwarfstd.org<br>
<b>Subject:</b> [Dwarf-discuss] DW_AT_frame_base<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Please help me interpret the DWARF spec regarding the DW_AT_frame_base. Quoted by spec v3, section 3.5.5, but similar wording in v4 and v5:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black">“A subroutine or entry point entry may also have a
</span><span style="font-size:11.5pt;color:blue">DW_AT_frame_base </span><span style="font-size:11.5pt;color:black">attribute, whose value is a location description that computes the “frame base” for the subroutine or entry point. If the location description
is a register operation, the given register contains the frame base address. If the location description is a DWARF expression, the result of evaluating that expression is the frame base address. Finally, for a location list, this interpretation applies to
each location expression contained in the list of location list entries.”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black">So what does “location description that is a register operation” mean here? Since the option of a DWARF expression block is covered by the second option, that rather suggests to me that “register
operation” is not a DWARF operation. I mean, the wording “if A is X, then Y. If A is P, then Q” usually means that P is distinct from X, right?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black">On a more practical note, I’m currently staring at a crash report with a DWARF attribute parsing failure. DWARF v3 Linux ELF binary, produced by NASM. DW_AT_frame_base, form DW_FORM_data4, value
0. The code assumes it’s a loclist pointer, but the binary doesn’t contain a loclist section.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black">Zero as a loclist pointer in a v3 binary could make sense. As a “register operation” – I’m not sure. Could be a compiler quirk, but I’m admitting the possibility that I’m misreading something.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.5pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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