Compute A-B when A or B is weak.

Similar to r235222, but for the weak symbol case.

In an "ideal" assembler/object format an expression would always refer to the
final value and A-B would only be computed from a section in the same
comdat as A and B with A and B strong.

Unfortunately that is not the case with debug info on ELF, so we need an
heuristic.  Since we need an heuristic, we may as well use the same one as
gas:

* call weak_sym : produces a relocation, even if in the same section.
* A - weak_sym and weak_sym -A: don't produce a relocation if we can
  compute it.

This fixes pr23272 and changes the fix of pr22815 to match what gas does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235227 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCMachObjectWriter.h

@@ -264,7 +264,6 @@ public:
 
   bool IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                               const MCSymbolData &DataA,
-                                              const MCSymbolData *DataB,
                                               const MCFragment &FB,
                                               bool InSet,
                                               bool IsPCRel) const override;

include/llvm/MC/MCObjectWriter.h

@@ -94,7 +94,6 @@ public:
 
   virtual bool IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                                       const MCSymbolData &DataA,
-                                                      const MCSymbolData *DataB,
                                                       const MCFragment &FB,
                                                       bool InSet,
                                                       bool IsPCRel) const;

lib/MC/ELFObjectWriter.cpp

@@ -275,7 +275,6 @@ class ELFObjectWriter : public MCObjectWriter {
     bool
     IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                            const MCSymbolData &DataA,
-                                           const MCSymbolData *DataB,
                                            const MCFragment &FB,
                                            bool InSet,
                                            bool IsPCRel) const override;
@@ -1669,13 +1668,15 @@ void ELFObjectWriter::WriteObject(MCAssembler &Asm,
 }
 
 bool ELFObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
-    const MCAssembler &Asm, const MCSymbolData &DataA,
+    const MCAssembler &Asm, const MCSymbolData &DataA, const MCFragment &FB,
-    const MCSymbolData *DataB, const MCFragment &FB, bool InSet,
+    bool InSet, bool IsPCRel) const {
-    bool IsPCRel) const {
+  if (IsPCRel) {
-  if (!InSet && (::isWeak(DataA) || (DataB && ::isWeak(*DataB))))
+    assert(!InSet);
+    if (::isWeak(DataA))
       return false;
-  return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
+  }
-      Asm, DataA, DataB, FB, InSet, IsPCRel);
+  return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(Asm, DataA, FB,
+                                                                InSet, IsPCRel);
 }
 
 bool ELFObjectWriter::isWeak(const MCSymbolData &SD) const {

lib/MC/MCAssembler.cpp

@@ -504,7 +504,7 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
       } else {
         const MCSymbolData &DataA = getSymbolData(SA);
         IsResolved = getWriter().IsSymbolRefDifferenceFullyResolvedImpl(
-            *this, DataA, nullptr, *DF, false, true);
+            *this, DataA, *DF, false, true);
       }
     }
   } else {

lib/MC/MCObjectWriter.cpp

@@ -36,13 +36,12 @@ bool MCObjectWriter::IsSymbolRefDifferenceFullyResolved(
     return false;
 
   return IsSymbolRefDifferenceFullyResolvedImpl(
-      Asm, DataA, &DataB, *DataB.getFragment(), InSet, false);
+      Asm, DataA, *DataB.getFragment(), InSet, false);
 }
 
 bool MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
-    const MCAssembler &Asm, const MCSymbolData &DataA,
+    const MCAssembler &Asm, const MCSymbolData &DataA, const MCFragment &FB,
-    const MCSymbolData *DataB, const MCFragment &FB, bool InSet,
+    bool InSet, bool IsPCRel) const {
-    bool IsPCRel) const {
   const MCSection &SecA = DataA.getSymbol().getSection();
   const MCSection &SecB = FB.getParent()->getSection();
   // On ELF and COFF  A - B is absolute if A and B are in the same section.

lib/MC/MachObjectWriter.cpp

@@ -669,13 +669,9 @@ void MachObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
   BindIndirectSymbols(Asm);
 }
 
-bool MachObjectWriter::
+bool MachObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
-IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
+    const MCAssembler &Asm, const MCSymbolData &DataA, const MCFragment &FB,
-                                       const MCSymbolData &DataA,
+    bool InSet, bool IsPCRel) const {
-                                       const MCSymbolData *DataB,
-                                       const MCFragment &FB,
-                                       bool InSet,
-                                       bool IsPCRel) const {
   if (InSet)
     return true;
 

lib/MC/WinCOFFObjectWriter.cpp

@@ -172,7 +172,6 @@ public:
 
   bool IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                               const MCSymbolData &DataA,
-                                              const MCSymbolData *DataB,
                                               const MCFragment &FB, bool InSet,
                                               bool IsPCRel) const override;
 
@@ -649,17 +648,16 @@ void WinCOFFObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
 }
 
 bool WinCOFFObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
-    const MCAssembler &Asm, const MCSymbolData &DataA,
+    const MCAssembler &Asm, const MCSymbolData &DataA, const MCFragment &FB,
-    const MCSymbolData *DataB, const MCFragment &FB, bool InSet,
+    bool InSet, bool IsPCRel) const {
-    bool IsPCRel) const {
   // MS LINK expects to be able to replace all references to a function with a
   // thunk to implement their /INCREMENTAL feature.  Make sure we don't optimize
   // away any relocations to functions.
   if ((((DataA.getFlags() & COFF::SF_TypeMask) >> COFF::SF_TypeShift) >>
        COFF::SCT_COMPLEX_TYPE_SHIFT) == COFF::IMAGE_SYM_DTYPE_FUNCTION)
     return false;
-  return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
+  return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(Asm, DataA, FB,
-      Asm, DataA, DataB, FB, InSet, IsPCRel);
+                                                                InSet, IsPCRel);
 }
 
 bool WinCOFFObjectWriter::isWeak(const MCSymbolData &SD) const {

test/MC/ELF/weak-diff.s

@@ -1,6 +1,10 @@
-// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t 2>&1 | FileCheck %s
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu < %s | llvm-readobj -r | FileCheck %s
 
-// CHECK: error: Cannot represent a subtraction with a weak symbol
+// CHECK:      Relocations [
+// CHECK-NEXT:   Section ({{.*}}) .rela.text {
+// CHECK-NEXT:     0x1D R_X86_64_PC32 f2 0xFFFFFFFFFFFFFFFC
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
 
 .weak f
 .weak g
@@ -10,3 +14,13 @@ g:
     nop
 
 .quad g - f
+
+
+.weak f2
+f2:
+    nop
+g2:
+    nop
+.quad g2 - f2
+.quad f2 - g2
+call f2

test/MC/ELF/weak-diff2.s

@@ -1,10 +0,0 @@
-// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t 2>&1 | FileCheck %s
-
-// CHECK: error: Cannot represent a subtraction with a weak symbol
-
-.weak f
-f:
-    nop
-g:
-    nop
-.quad g - f