[PATCH] D87407: [WebAssembly][MC] Fix computation of relative symbol offset

[Dominic Chen via Phabricator via llvm-commits]

ddcc created this revision.
ddcc added reviewers: sbc100, aardappel.
Herald added subscribers: ecnelises, sunfish, hiraditya, jgravelle-google, dschuff.
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Herald added a subscriber: aheejin.

For relative symbols, add its offset when computing relocation value.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D87407

Files:
  llvm/lib/MC/WasmObjectWriter.cpp
  llvm/test/MC/WebAssembly/alias-relative.ll


Index: llvm/test/MC/WebAssembly/alias-relative.ll
===================================================================
--- /dev/null
+++ llvm/test/MC/WebAssembly/alias-relative.ll
@@ -0,0 +1,34 @@
+; RUN: llc -filetype=obj %s -o - | obj2yaml | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+define void @f() {
+  ret void
+}
+
+ at 0 = private constant { [2 x i8*] } { [2 x i8*] [i8* null, i8* bitcast (void ()* @f to i8*)] }, align 4
+
+ at alias_f = private alias i8*, getelementptr inbounds ({ [2 x i8*] }, { [2 x i8*] }* @0, i32 0, i32 0, i32 1)
+
+define void @_start() {
+  store i8* null, i8** getelementptr inbounds (i8*, i8** @alias_f, i32 0), align 4
+  ret void
+}
+
+; CHECK:  - Type:            DATACOUNT
+; CHECK-NEXT:    Count:           1
+; CHECK-NEXT:  - Type:            CODE
+; CHECK-NEXT:    Relocations:
+; CHECK-NEXT:      - Type:            R_WASM_MEMORY_ADDR_LEB
+; CHECK-NEXT:        Index:           2
+; CHECK-NEXT:        Offset:          0x0000000C
+; CHECK-NEXT:    Functions:
+; CHECK-NEXT:      - Index:           0
+; CHECK-NEXT:        Locals:          []
+; CHECK-NEXT:        Body:            0B
+; CHECK-NEXT:      - Index:           1
+; CHECK-NEXT:        Locals:          []
+; CHECK-NEXT:        Body:            41004100360284808080000B
+
+; incorrect code will generate 41004100360280808080000B
+; wasm-ld: warning: unexpected existing value for R_WASM_MEMORY_ADDR_LEB: existing=0 expected=4
Index: llvm/lib/MC/WasmObjectWriter.cpp
===================================================================
--- llvm/lib/MC/WasmObjectWriter.cpp
+++ llvm/lib/MC/WasmObjectWriter.cpp
@@ -592,16 +592,17 @@
   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB64:
   case wasm::R_WASM_MEMORY_ADDR_I32:
   case wasm::R_WASM_MEMORY_ADDR_I64: {
-    // Provisional value is address of the global
+    // Provisional value is address of the global plus the offset
     const MCSymbolWasm *Base =
         cast<MCSymbolWasm>(Layout.getBaseSymbol(*RelEntry.Symbol));
     // For undefined symbols, use zero
     if (!Base->isDefined())
       return 0;
-    const wasm::WasmDataReference &Ref = DataLocations[Base];
-    const WasmDataSegment &Segment = DataSegments[Ref.Segment];
+    const wasm::WasmDataReference &BaseRef = DataLocations[Base];
+    const wasm::WasmDataReference &SymRef = DataLocations[RelEntry.Symbol];
+    const WasmDataSegment &Segment = DataSegments[BaseRef.Segment];
     // Ignore overflow. LLVM allows address arithmetic to silently wrap.
-    return Segment.Offset + Ref.Offset + RelEntry.Addend;
+    return Segment.Offset + BaseRef.Offset + SymRef.Offset + RelEntry.Addend;
   }
   default:
     llvm_unreachable("invalid relocation type");
@@ -1225,8 +1226,11 @@
     // Register types for all functions, including those with private linkage
     // (because wasm always needs a type signature).
     if (WS.isFunction()) {
-      const MCSymbolWasm *Base = cast<MCSymbolWasm>(Layout.getBaseSymbol(S));
-      registerFunctionType(*Base);
+      auto *BS = Layout.getBaseSymbol(S);
+      if (!BS)
+        report_fatal_error(Twine(S.getName()) +
+                           ": absolute addressing not supported!");
+      registerFunctionType(*cast<MCSymbolWasm>(BS));
     }
 
     if (WS.isEvent())
@@ -1516,7 +1520,11 @@
 
     assert(S.isDefined());
 
-    const MCSymbolWasm *Base = cast<MCSymbolWasm>(Layout.getBaseSymbol(S));
+    auto *BS = Layout.getBaseSymbol(S);
+    if (!BS)
+      report_fatal_error(Twine(S.getName()) +
+                         ": absolute addressing not supported!");
+    const MCSymbolWasm *Base = cast<MCSymbolWasm>(BS);
 
     // Find the target symbol of this weak alias and export that index
     const auto &WS = static_cast<const MCSymbolWasm &>(S);


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