[WebAssembly][MC] Simplify WasmObjectWriter::recordRelocation. NFC.

[sbc100]

Summary

WebAssembly doesn't support PC relative relocation or relocation
expressions that can't be reduced to single symbol.

The only support for we have for fixups involving two symbols are when
both symbols are defined and withing the same section. In this case
evaluateFixup will already have evaluated to the expression before
calling recordRelocation.

[sbc100]

I mistakenly landed this change already, so this is post-commit review. Happy to revert if needed.

[dschuff]

So it looks like this change is just that we now report an error where it asserted before?

[sbc100]

In D66455#1637385, @dschuff wrote:

So it looks like this change is just that we now report an error where it asserted before?

Also removes a bunch of redundant code (which I think was originally copied from the ELF file writer).

[sbc100]

This landed as rL369317

llvm/lib/MC/WasmObjectWriter.cpp

 }
 
 void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         const MCFragment *Fragment,
                                         const MCFixup &Fixup, MCValue Target,
                                         uint64_t &FixedValue) {
   MCAsmBackend &Backend = Asm.getBackend();
-  bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
-                 MCFixupKindInfo::FKF_IsPCRel;
+
+  // The WebAssembly backend should never generate FKF_IsPCRel fixups
+  assert(!(Backend.getFixupKindInfo(Fixup.getKind()).Flags &
+           MCFixupKindInfo::FKF_IsPCRel));
+
   const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());
   uint64_t C = Target.getConstant();
   uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
   MCContext &Ctx = Asm.getContext();
 
   // The .init_array isn't translated as data, so don't do relocations in it.
   if (FixupSection.getSectionName().startswith(".init_array"))
     return;
 
   if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
-    assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
-           "Should not have constructed this");
-
-    // Let A, B and C being the components of Target and R be the location of
-    // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
-    // If it is pcrel, we want to compute (A - B + C - R).
-
-    // In general, Wasm has no relocations for -B. It can only represent (A + C)
-    // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
-    // replace B to implement it: (A - R - K + C)
-    if (IsPCRel) {
-      Ctx.reportError(
-          Fixup.getLoc(),
-          "No relocation available to represent this relative expression");
-      return;
-    }
-
-    const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
-
-    if (SymB.isUndefined()) {
-      Ctx.reportError(Fixup.getLoc(),
-                      Twine("symbol '") + SymB.getName() +
-                          "' can not be undefined in a subtraction expression");
-      return;
-    }
+    // To get here the A - B expression must have failed evaluateAsRelocatable.
+    // This means either A or B must be undefined and in WebAssembly we can't
+    // support either of those cases.
+    const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
+            "': unsupported subtraction expression used in relocation.");
 
-    assert(!SymB.isAbsolute() && "Should have been folded");
-    const MCSection &SecB = SymB.getSection();
-    if (&SecB != &FixupSection) {
-      Ctx.reportError(Fixup.getLoc(),
-                      "Cannot represent a difference across sections");
-      return;
-    }
-
-    uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
-    uint64_t K = SymBOffset - FixupOffset;
-    IsPCRel = true;
-    C -= K;
-  }
-
   // We either rejected the fixup or folded B into C at this point.
   const MCSymbolRefExpr *RefA = Target.getSymA();
-  const auto *SymA = RefA ? cast<MCSymbolWasm>(&RefA->getSymbol()) : nullptr;
+  const auto *SymA = cast<MCSymbolWasm>(&RefA->getSymbol());
 
-  if (SymA && SymA->isVariable()) {
+  if (SymA->isVariable()) {
     const MCExpr *Expr = SymA->getVariableValue();
     const auto *Inner = cast<MCSymbolRefExpr>(Expr);
     if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
       llvm_unreachable("weakref used in reloc not yet implemented");
   }
 
   // Put any constant offset in an addend. Offsets can be negative, and
   // LLVM expects wrapping, in contrast to wasm's immediates which can't
   // be negative and don't wrap.
   FixedValue = 0;
 
   unsigned Type = TargetObjectWriter->getRelocType(Target, Fixup);
-  assert(!IsPCRel);
-  assert(SymA);
 
   // Absolute offset within a section or a function.
   // Currently only supported for for metadata sections.
   // See: test/MC/WebAssembly/blockaddress.ll
   if (Type == wasm::R_WASM_FUNCTION_OFFSET_I32 ||
       Type == wasm::R_WASM_SECTION_OFFSET_I32) {
     if (!FixupSection.getKind().isMetadata())
       report_fatal_error("relocations for function or section offsets are "

llvm/test/MC/WebAssembly/bad-fixup-expr.s

+// RUN: not llvm-mc -filetype=obj -triple wasm32 %s -o /dev/null 2>&1 | FileCheck %s
+
+  .section    .data.foo,"",@
+foo:
+  .int8       1
+  .size       foo, 1
+foo_other:
+  .int8       1
+  .size       foo_other, 1
+
+  .section    .data.bar,"",@
+bar:
+  .int8       1
+  .size       bar, 1
+
+  .text
+  .section    .text.main,"",@
+main:
+  .functype   main () -> (i32)
+// Expressions involving symbols within the same sections can be evaluated
+// prior to writing the object file.
+// CHECK-NOT: foo
+  i32.const foo-foo_other+2
+  i32.const foo_other-foo-10
+
+// CHECK: 'bar': unsupported subtraction expression used in relocation
+  i32.const foo-bar
+// CHECK: 'undef_baz': unsupported subtraction expression used in relocation
+  i32.const foo-undef_baz
+// CHECK: 'foo': unsupported subtraction expression used in relocation
+  i32.const undef_baz-foo
+  end_function