[llvm-commits] [patch] Add MCObjectFormat. Fix "a + (b - c)" for ELF

[Michael Spencer]

On Sat, Oct 16, 2010 at 12:21 AM, Rafael Espindola <espindola at google.com> wrote:
> Now the patch is attached.
>
> Cheers,
> --
> Rafael Ávila de Espíndola
>

Looks like an improvement to me. The only thing I wonder is if
IsFixupFullyResolved should be moved there. Seems like a similar
query.

Inline comments below.

diff --git a/lib/MC/MCExpr.cpp b/lib/MC/MCExpr.cpp
index e9a6062..3524b80 100644
--- a/lib/MC/MCExpr.cpp
+++ b/lib/MC/MCExpr.cpp
@@ -14,6 +14,7 @@
 #include "llvm/MC/MCAsmLayout.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCObjectFormat.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/Debug.h"
@@ -237,7 +238,8 @@ bool MCExpr::EvaluateAsAbsolute(int64_t &Res,
const MCAsmLayout *Layout) const {
   return true;
 }

-static bool EvaluateSymbolicAdd(const MCValue &LHS,const
MCSymbolRefExpr *RHS_A,
+static bool EvaluateSymbolicAdd(const MCAsmLayout *Layout, bool InSet,
+                                const MCValue &LHS,const
MCSymbolRefExpr *RHS_A,
                                 const MCSymbolRefExpr *RHS_B, int64_t RHS_Cst,
                                 MCValue &Res) {
   // We can't add or subtract two symbols.
@@ -255,12 +257,40 @@ static bool EvaluateSymbolicAdd(const MCValue
&LHS,const MCSymbolRefExpr *RHS_A,
     if (!A)
       return false;
   }
+
+  // Absolutize symbol differences between defined symbols when we have a
+  // layout object and the target requests it.
+
+  if (Layout && A && B) {
+    const MCSymbol &SA = A->getSymbol();
+    const MCSymbol &SB = B->getSymbol();
+    const MCObjectFormat &F =
+      Layout->getAssembler().getBackend().getObjectFormat();
+    if (SA.isDefined() && SB.isDefined() && F.isAbsolute(InSet, SA, SB)) {
+      const MCAssembler &Asm = Layout->getAssembler();
+      MCSymbolData &AD = Asm.getSymbolData(A->getSymbol());
+      MCSymbolData &BD = Asm.getSymbolData(B->getSymbol());
+      Res = MCValue::get(+ Layout->getSymbolAddress(&AD)

I have finally found a useful application of unary +.

+                         - Layout->getSymbolAddress(&BD)
+                         + LHS.getConstant()
+                         + RHS_Cst);
+      return true;
+    }
+  }
+
+
   Res = MCValue::get(A, B, LHS.getConstant() + RHS_Cst);
   return true;
 }
diff --git a/lib/MC/MCObjectFormat.cpp b/lib/MC/MCObjectFormat.cpp
new file mode 100644
index 0000000..9c14179
--- /dev/null
+++ b/lib/MC/MCObjectFormat.cpp
@@ -0,0 +1,31 @@
+//===- lib/MC/MCObjectFormat.cpp - MCObjectFormat implementation
----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/MC/MCObjectFormat.h"
+#include "llvm/MC/MCSymbol.h"
+
+using namespace llvm;
+
+bool MCELFObjectFormat::isAbsolute(bool IsSet, const MCSymbol &A,
+                                   const MCSymbol &B) const {
+  // On ELF A - B is absolute if A and B are in the same section.
+  return &A.getSection() == &B.getSection();
+}
+
+bool MCMachOObjectFormat::isAbsolute(bool IsSet, const MCSymbol &A,
+                                     const MCSymbol &B) const  {
+  // On MachO A - B is absolute only if in a set.
+  return IsSet;
+}
+
+bool MCCOFFObjectFormat::isAbsolute(bool IsSet, const MCSymbol &A,
+                                    const MCSymbol &B) const  {
+  // FIXME: Is this correct?

This should be the same as the ELF case.

+  return false;
+}

- Michael Spencer