[llvm] [Sframe] Support cfi_escape directives compatibly with gnu-gas (PR #161927)

[Fangrui Song via llvm-commits]

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@@ -211,8 +213,152 @@ class SFrameEmitterImpl {
     return true;
   }
 
+  // Technically, the escape data could be anything, but it is commonly a dwarf
+  // CFI program. Even then, it could contain an arbitrarily complicated Dwarf
+  // expression. Following gnu-gas, look for certain common cases that could
+  // invalidate an FDE, emit a warning for those sequences, and don't generate
+  // an FDE in those cases. Allow any that are known safe. It is likely that
+  // more thorough test cases could refine this code, but it handles the most
+  // important ones compatibly with gas.
+  // Returns true if the CFI escape sequence is safe for sframes.
+  bool isCFIEscapeSafe(SFrameFDE &FDE, const SFrameFRE &FRE,
+                       const MCCFIInstruction &CFI) {
+    const MCAsmInfo *AI = Streamer.getContext().getAsmInfo();
+    DWARFDataExtractorSimple data(CFI.getValues(), AI->isLittleEndian(),
+                                  AI->getCodePointerSize());
+
+    // Normally, both alignment factors are extracted from the enclosing Dwarf
+    // FDE or CIE. We don't have one here. Alignments are used for scaling
+    // factors for ops like CFA_def_cfa_offset_sf. But this particular function
+    // is only interested in registers.
+    dwarf::CFIProgram P(/* CodeAlignmentFactor */ 1,
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MaskRay wrote:

The standard style is `/*CodeAlignmentFactor=*/1` (no space). This is accepted by both clang-format and a clang-tidy check.

https://github.com/llvm/llvm-project/pull/161927