[llvm] [CFIFixup] Allow function prologues to span more than one basic block (PR #68984)

[Fangrui Song via llvm-commits]

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@@ -10,20 +10,25 @@
 // This pass inserts the necessary  instructions to adjust for the inconsistency
 // of the call-frame information caused by final machine basic block layout.
 // The pass relies in constraints LLVM imposes on the placement of
-// save/restore points (cf. ShrinkWrap):
-// * there is a single basic block, containing the function prologue
+// save/restore points (cf. ShrinkWrap) and has certain preconditions about
+// placement of CFI instructions:
+// * for any two CFI instructions of the function prologue one dominates
+//   and is post-dominated by the other
 // * possibly multiple epilogue blocks, where each epilogue block is
 //   complete and self-contained, i.e. CSR restore instructions (and the
 //   corresponding CFI instructions are not split across two or more blocks.
-// * prologue and epilogue blocks are outside of any loops
-// Thus, during execution, at the beginning and at the end of each basic block
-// the function can be in one of two states:
+// * CFI instructions are not contained in any loops
+// Thus, during execution, at the beginning and at the end of each basic block,
+// following the prologue, the function can be in one of two states:
 //  - "has a call frame", if the function has executed the prologue, and
 //    has not executed any epilogue
 //  - "does not have a call frame", if the function has not executed the
 //    prologue, or has executed an epilogue
 // which can be computed by a single RPO traversal.
 
+// The location of the prologue is determined by finding the first block in the
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MaskRay wrote:

This is not accurate. `PrologueBlock` is the last basic block that satisfies the condition.

https://github.com/llvm/llvm-project/pull/68984