https://bugs.llvm.org/show_bug.cgi?id=43926
Bug ID: 43926
Summary: [call site info / entry values] DW_AT_call_target
describes clobbered register
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: DebugInfo
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected],
[email protected]
When describing an indirect call, llvm may specify a clobbered register as the
call target.
Example:
```
caller:
...
mov (%rdi), %rax
callq *%rax
->
DW_TAG_call_site
DW_AT_call_target (DW_OP_reg0 RAX)
DW_AT_call_return_pc ...
```
Here, %rax is not preserved by the callee. This means that the debugger cannot
figure out the actual call target, and therefore, I believe it cannot safely
evaluate entry values in the eventual callee.
Example: `callq *%rax` may branch to `tail_calling_helper`, which can tail call
`eventual_callee`; alternatively, `callq *%rax` can branch to `eventual_callee`
immediately. The debugger simply does not know the call target, so it does not
know which function to search for call site parameter infos.
How does gcc handle this? Should llvm attempt to "work backwards" to find a
non-clobbered description of the call target (perhaps using the existing
`describeLoadedValue` API)?
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