[llvm] r369317 - [WebAssembly][MC] Simplify WasmObjectWriter::recordRelocation. NFC.

[Sam Clegg via llvm-commits]

Author: sbc
Date: Mon Aug 19 17:33:50 2019
New Revision: 369317

URL: http://llvm.org/viewvc/llvm-project?rev=369317&view=rev
Log:
[WebAssembly][MC] Simplify WasmObjectWriter::recordRelocation. NFC.

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.

Added:
    llvm/trunk/test/MC/WebAssembly/bad-fixup-expr.s
Modified:
    llvm/trunk/lib/MC/WasmObjectWriter.cpp

Modified: llvm/trunk/lib/MC/WasmObjectWriter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/WasmObjectWriter.cpp?rev=369317&r1=369316&r2=369317&view=diff
==============================================================================
--- llvm/trunk/lib/MC/WasmObjectWriter.cpp (original)
+++ llvm/trunk/lib/MC/WasmObjectWriter.cpp Mon Aug 19 17:33:50 2019
@@ -427,8 +427,11 @@ void WasmObjectWriter::recordRelocation(
                                         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();
@@ -439,51 +442,22 @@ void WasmObjectWriter::recordRelocation(
     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;
-    }
-
+    // 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());
-
-    if (SymB.isUndefined()) {
-      Ctx.reportError(Fixup.getLoc(),
-                      Twine("symbol '") + SymB.getName() +
-                          "' can not be undefined in a subtraction expression");
-      return;
-    }
-
-    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;
+    Ctx.reportError(
+        Fixup.getLoc(),
+        Twine("symbol '") + SymB.getName() +
+            "': unsupported subtraction expression used in relocation.");
+    return;
   }
 
   // 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)
@@ -496,8 +470,6 @@ void WasmObjectWriter::recordRelocation(
   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.

Added: llvm/trunk/test/MC/WebAssembly/bad-fixup-expr.s
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/WebAssembly/bad-fixup-expr.s?rev=369317&view=auto
==============================================================================
--- llvm/trunk/test/MC/WebAssembly/bad-fixup-expr.s (added)
+++ llvm/trunk/test/MC/WebAssembly/bad-fixup-expr.s Mon Aug 19 17:33:50 2019
@@ -0,0 +1,32 @@
+// 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