[AIX] Implement function descriptor on SDAG

Summary:
(1) Function descriptor on AIX
On AIX, a called routine may have 2 distinct symbols associated with it:
 * A function descriptor (Name)
 * A function entry point (.Name)

The descriptor structure on AIX is the same as those in the ELF V1 ABI:
 * The address of the entry point of the function.
 * The TOC base address for the function.
 * The environment pointer.

The descriptor symbol uses the same name as the source level function in C.
The function entry point is analogous to the symbol we would generate for a
 function in a non-descriptor-based ABI, except that it is renamed by
prepending a ".".

Which symbol gets referenced depends on the context:
 * Taking the address of the function references the descriptor symbol.
 * Calling the function references the entry point symbol.

(2) Speaking of implementation on AIX, for direct function call target, we
 create proper MCSymbol SDNode(e.g . ".foo") while constructing SDAG to
 replace original TargetGlobalAddress SDNode. Then down the path, we can
 take advantage of this MCSymbol.

Patch by: Xiangling_L

Reviewed by: sfertile, hubert.reinterpretcast, jasonliu, syzaara

Differential Revision: https://reviews.llvm.org/D62532

llvm-svn: 362735

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index d887ed73c44..72c4c0d8293 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1877,6 +1877,13 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
     return false;
 
   case Instruction::Call:
+    // On AIX, call lowering uses the DAG-ISEL path currently so that the
+    // callee of the direct function call instruction will be mapped to the
+    // symbol for the function's entry point, which is distinct from the
+    // function descriptor symbol. The latter is the symbol whose XCOFF symbol
+    // name is the C-linkage name of the source level function.
+    if (TM.getTargetTriple().isOSAIX())
+      return false;
     return selectCall(I);
 
   case Instruction::BitCast: