MC/Mach-O: Implement "absolutizing" semantics of .set, by evaluating the assembly time value of variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98241 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCAssembler.cpp

@@ -539,10 +539,10 @@ public:
     if (Target.isAbsolute()) { // constant
       // SymbolNum of 0 indicates the absolute section.
       //
-      // FIXME: When is this generated?
+      // FIXME: Currently, these are never generated (see code below). I cannot
+      // find a case where they are actually emitted.
       Type = RIT_Vanilla;
       Value = 0;
-      llvm_unreachable("FIXME: Not yet implemented!");
     } else {
       const MCSymbol *Symbol = Target.getSymA();
       MCSymbolData *SD = &Asm.getSymbolData(*Symbol);
@@ -572,6 +572,12 @@ public:
     if (IsPCRel)
       Fixup.FixedValue -= Address;
 
+    // If the target evaluates to a constant, we don't need a relocation. This
+    // occurs with absolutized expressions which are not resolved to constants
+    // until after relaxation.
+    if (Target.isAbsolute())
+      return;
+
     // If this fixup is a vanilla PC relative relocation for a local label, we
     // don't need a relocation.
     //

lib/MC/MCExpr.cpp

@@ -8,11 +8,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCAsmLayout.h"
+#include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetAsmBackend.h"
 using namespace llvm;
 
 void MCExpr::print(raw_ostream &OS) const {
@@ -187,8 +190,23 @@ bool MCExpr::EvaluateAsRelocatable(MCValue &Res, MCAsmLayout *Layout) const {
     const MCSymbol &Sym = cast<MCSymbolRefExpr>(this)->getSymbol();
 
     // Evaluate recursively if this is a variable.
-    if (Sym.isVariable())
-      return Sym.getValue()->EvaluateAsRelocatable(Res, Layout);
+    if (Sym.isVariable()) {
+      if (!Sym.getValue()->EvaluateAsRelocatable(Res, Layout))
+        return false;
+
+      // Absolutize symbol differences when we have a layout object and the
+      // target requests it.
+      if (Layout && Res.getSymB() &&
+          Layout->getAssembler().getBackend().hasAbsolutizedSet()) {
+        MCSymbolData &A = Layout->getAssembler().getSymbolData(*Res.getSymA());
+        MCSymbolData &B = Layout->getAssembler().getSymbolData(*Res.getSymB());
+        Res = MCValue::get(+ A.getFragment()->getAddress() + A.getOffset()
+                           - B.getFragment()->getAddress() - B.getOffset()
+                           + Res.getConstant());
+      }
+
+      return true;
+    }
 
     Res = MCValue::get(&Sym, 0, 0);
     return true;

test/MC/MachO/absolutize.s

@@ -0,0 +1,71 @@
+// RUN: llvm-mc -triple i386-apple-darwin9 %s -filetype=obj -o - | macho-dump | FileCheck %s
+
+// CHECK: # Relocation 0
+// CHECK: (('word-0', 0xa0000028),
+// CHECK:  ('word-1', 0x2b)),
+// CHECK: # Relocation 1
+// CHECK: (('word-0', 0xa4000020),
+// CHECK:  ('word-1', 0x37)),
+// CHECK: # Relocation 2
+// CHECK: (('word-0', 0xa1000000),
+// CHECK:  ('word-1', 0x33)),
+// CHECK: # Relocation 3
+// CHECK: (('word-0', 0xa4000018),
+// CHECK:  ('word-1', 0x33)),
+// CHECK: # Relocation 4
+// CHECK: (('word-0', 0xa1000000),
+// CHECK:  ('word-1', 0x2f)),
+// CHECK: # Relocation 5
+// CHECK: (('word-0', 0xa4000010),
+// CHECK:  ('word-1', 0x2b)),
+// CHECK: # Relocation 6
+// CHECK: (('word-0', 0xa1000000),
+// CHECK:  ('word-1', 0x2f)),
+// CHECK-NEXT: ])
+
+_text_a:
+        xorl %eax,%eax
+_text_b:
+        xorl %eax,%eax
+Ltext_c:
+        xorl %eax,%eax
+Ltext_d:        
+        xorl %eax,%eax
+        
+        movl $(_text_a - _text_b), %eax
+Ltext_expr_0 = _text_a - _text_b
+        movl $(Ltext_expr_0), %eax
+
+        movl $(Ltext_c - _text_b), %eax
+Ltext_expr_1 = Ltext_c - _text_b
+        movl $(Ltext_expr_1), %eax
+
+        movl $(Ltext_d - Ltext_c), %eax
+Ltext_expr_2 = Ltext_d - Ltext_c
+        movl $(Ltext_expr_2), %eax
+
+        movl $(_text_a + Ltext_expr_0), %eax
+
+        .data
+_data_a:
+        .long 0
+_data_b:
+        .long 0
+Ldata_c:
+        .long 0
+Ldata_d:        
+        .long 0
+        
+        .long _data_a - _data_b
+Ldata_expr_0 = _data_a - _data_b
+        .long Ldata_expr_0
+
+        .long Ldata_c - _data_b
+Ldata_expr_1 = Ldata_c - _data_b
+        .long Ldata_expr_1
+
+        .long Ldata_d - Ldata_c
+Ldata_expr_2 = Ldata_d - Ldata_c
+        .long Ldata_expr_2
+
+        .long _data_a + Ldata_expr_0