[libcxx-commits] [PATCH] D158655: [libunwind][AIX] Fix problem with stepping up from a leaf function when unwinding started in a signal handler

[Hubert Tong via Phabricator via libcxx-commits]

hubert.reinterpretcast added inline comments.


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Comment at: libunwind/src/UnwindCursor.hpp:2324
   // lastStack points to the stack frame of the next routine up.
   pint_t lastStack = *(reinterpret_cast<pint_t *>(registers.getSP()));
 
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This only covers the case when `stores_bc` is `1` in the traceback table.


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Comment at: libunwind/src/UnwindCursor.hpp:2398
+      // sigcontext no longer points to the caller of the leaf function.
+      // However, in this case the compiler will buy a stack frame and save
+      // the original LR value in the link area and therefore, the LR from
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The comment should say nothing about buying a stack frame. The link area is usable by a function for saving the return address even when said function does not increment the SP. As @stephenpeckham pointed out "offline", there can be even be non-leaf functions that do not buy a stack frame (e.g., because the functions they call are known to not write to the stack).


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Comment at: libunwind/src/UnwindCursor.hpp:2404
+                                 reinterpret_cast<void *>(returnAddress));
+    } else
+      // Otherwise, use the LR value in the stack link area.
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hubert.reinterpretcast wrote:
> https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements now states that braces should be used for each part of an if/else chain when one part needs braces.
Might be good to explicitly set `returnAddress` to `0` if `lastStack` is `0`. That is, make this `else if (lastStack)`.


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Comment at: libunwind/src/UnwindCursor.hpp:2404-2406
+    } else
+      // Otherwise, use the LR value in the stack link area.
+      returnAddress = reinterpret_cast<pint_t *>(lastStack)[2];
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https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements now states that braces should be used for each part of an if/else chain when one part needs braces.


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Comment at: libunwind/src/UnwindCursor.hpp:2501-2504
+    // Restore the SP and move one frame up if it is non-leaf function,
+    // or the function is '__start', or LR is not set in the current context.
+    if (TBTable->tb.saves_lr || !lastStack || !sigContextLRValue)
+      newRegisters.setSP(lastStack);
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If `lastStack` truly "points to the stack frame of the next routine up" (as claimed where it is declared), then it is fine to restore the SP even given a leaf function.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D158655/new/

https://reviews.llvm.org/D158655