[llvm-commits] [llvm] r77855 - /llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp

[Chris Lattner]

Author: lattner
Date: Sat Aug  1 22:59:56 2009
New Revision: 77855

URL: http://llvm.org/viewvc/llvm-project?rev=77855&view=rev
Log:
Fix some fixme's in #if 0'd code by making it dependent on the structural
behavior of the LSDA section instead of on some random target hook that
needs to be kept in synch with other points of truth.

Modified:
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp?rev=77855&r1=77854&r2=77855&view=diff

==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp Sat Aug  1 22:59:56 2009
@@ -557,40 +557,45 @@
   Asm->EOL("LPStart format (DW_EH_PE_omit)");
 
 #if 0
-  // FIXME: This should default to what the system wants, not just "absptr".
   if (TypeInfos.empty() && FilterIds.empty()) {
+    // If there are no typeinfos or filters, there is nothing to emit, optimize
+    // by specifying the "omit" encoding.
     Asm->EmitInt8(dwarf::DW_EH_PE_omit);
     Asm->EOL("TType format (DW_EH_PE_omit)");
   } else {
-    // FIXME: Instead of using "PreferredEHDataFormat", we should use a simple
-    // approach to determine what needs to happen.  Basically, if the target
-    // wants the LSDA to be emitted into a read-only segment (like .text) then
-    // (unless in static mode) it can't output direct pointers to the typeinfo
-    // objects, which may be in an arbitrary locations.  Instead, it has to use
-    // and indirect stub pointer to get to the typeinfo.
+    // Okay, we have actual filters or typeinfos to emit.  As such, we need to
+    // pick a type encoding for them.  We're about to emit a list of pointers to
+    // typeinfo objects at the end of the LSDA.  However, unless we're in static
+    // mode, this reference will require a relocation by the dynamic linker.
     //
-    // If the target wants to dump the LSDA's into a segment writable by the
-    // dynamic linker, then it can just use a normal pointer, and the dynamic
-    // linker will fix it up.
-    
-    // TODO: Replace the getDwarfExceptionSection() callback on TAI with a new
-    // getLSDASection() method on TLOF.  Merge and sanitize the implementations,
-    // and figure out what the ".gcc_except_table" directive expands to on elf
-    // systems.
-    
-    // 
-    //if (LSDASection->isWritable()) {
-    //Asm->EmitInt8(DW_EH_PE_absptr);
-    //} else {
-    //Asm->EmitInt8(DW_EH_PE_pcrel | DW_EH_PE_indirect | DW_EH_PE_sdata4);
-    //}
-    
-    Asm->EmitInt8(TAI->PreferredEHDataFormat());
-    
-    
-    // FIXME: The comment here should correspond with what PreferredEHDataFormat
-    // returned.
-    Asm->EOL("TType format (DW_EH_PE_xxxxx)");
+    // Because of this, we have a couple of options:
+    //   1) If we are in -static mode, we can always use an absolute reference
+    //      from the LSDA, because the static linker will resolve it.
+    //   2) Otherwise, if the LSDA section is writable, we can output the direct
+    //      reference to the typeinfo and allow the dynamic linker to relocate
+    //      it.  Since it is in a writable section, the dynamic linker won't
+    //      have a problem.
+    //   3) Finally, if we're in PIC mode and the LDSA section isn't writable,
+    //      we need to use some form of indirection.  For example, on Darwin,
+    //      we can output a statically-relocatable reference to a dyld stub. The
+    //      offset to the stub is constant, but the contents are in a section
+    //      that is updated by the dynamic linker.  This is easy enough, but we
+    //      need to tell the personality function of the unwinder to indirect
+    //      through the dyld stub.
+    //
+    // FIXME: When this is actually implemented, we'll have to emit the stubs
+    // somewhere.  This predicate should be moved to a shared location that is
+    // in target-independent code.
+    //
+    if (LSDASection->isWritable() ||
+        Asm->TM.getRelocationModel() == Reloc::Static) {
+      Asm->EmitInt8(DW_EH_PE_absptr);
+      Asm->EOL("TType format (DW_EH_PE_absptr)");
+    } else {
+      Asm->EmitInt8(DW_EH_PE_pcrel | DW_EH_PE_indirect | DW_EH_PE_sdata4);
+      Asm->EOL("TType format (DW_EH_PE_pcrel | DW_EH_PE_indirect"
+               " | DW_EH_PE_sdata4)");
+    }
     Asm->EmitULEB128Bytes(TypeOffset);
     Asm->EOL("TType base offset");
   }