Fix aliases to thumbfunc-based exprs to be thumbfunc

[eugenis]

Summary

If F is a Thumb function symbol, and G = F + const, and G is a
function symbol, then G is Thumb. Because what else could it be?

This is how ControlFlowIntegrity emits jump tables - a function with inline asm that's a bunch of jump instructions, and a set of aliases inside that function. If a function is thumb, we want the aliases to be thumb as well.

[pcc]

LGTM

llvm/trunk/lib/MC/MCAssembler.cpp

 
 bool MCAssembler::isThumbFunc(const MCSymbol *Symbol) const {
   if (ThumbFuncs.count(Symbol))
     return true;
 
   if (!Symbol->isVariable())
     return false;
 
-  // FIXME: It looks like gas supports some cases of the form "foo + 2". It
-  // is not clear if that is a bug or a feature.
   const MCExpr *Expr = Symbol->getVariableValue();
-  const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr);
+
+  MCValue V;
+  if (!Expr->evaluateAsRelocatable(V, nullptr, nullptr))
+    return false;
+
+  if (V.getSymB() || V.getRefKind() != MCSymbolRefExpr::VK_None)
+    return false;
+
+  const MCSymbolRefExpr *Ref = V.getSymA();
   if (!Ref)
     return false;
 
   if (Ref->getKind() != MCSymbolRefExpr::VK_None)
     return false;
 
   const MCSymbol &Sym = Ref->getSymbol();
   if (!isThumbFunc(&Sym))

llvm/trunk/test/MC/ARM/elf-thumbfunc.s

 @@ test st_value bit 0 of thumb function
 @ RUN: llvm-mc %s -triple=thumbv7-linux-gnueabi -filetype=obj -o - | \
 @ RUN: llvm-readobj -t | FileCheck %s
   .syntax unified
   .text
   .globl  foo
   .align  2
   .code 16
   .thumb_func
   .type foo,%function
 foo:
   bx  lr
 
   .global bar
 bar = foo
 
+  .global baz
+baz = foo + 2
+
 @@ make sure foo and bar are thumb function: bit 0 = 1 (st_value)
 @CHECK:        Symbol {
 @CHECK:          Name: bar
 @CHECK-NEXT:     Value: 0x1
 @CHECK-NEXT:     Size: 0
 @CHECK-NEXT:     Binding: Global
 @CHECK-NEXT:     Type: Function
 
 @CHECK:        Symbol {
+@CHECK:          Name: baz
+@CHECK-NEXT:     Value: 0x3
+@CHECK-NEXT:     Size: 0
+@CHECK-NEXT:     Binding: Global
+@CHECK-NEXT:     Type: Function
+
+@CHECK:        Symbol {
 @CHECK:          Name: foo
 @CHECK-NEXT:     Value: 0x1
 @CHECK-NEXT:     Size: 0
 @CHECK-NEXT:     Binding: Global
 @CHECK-NEXT:     Type: Function