[MC][WebAssembly] Fix provisional values for data alias relocations

[sbc100]

Summary

When calculating the symbol offsets to write as provisitonal values
in object files we are only interested in the offset of the symbol
itself. For aliases this offset already includes the offset of the
base symbol.

The test in question was added back in https://reviews.llvm.org/D87407
but I believe the expectations here were incorrect. sym_a lives
at offset 4 and sym_b lives 4 bytes into that (should be 8).

The addresses of the 3 symbosl in this object file are:

foo : 0
sym_a: 4
sym_b: 8

[ddcc]

Makes sense to me, since the offsets now match the symbol table checked earlier in the test. But I'm not really familiar with symbol processing, and I don't have a local WAsm toolchain to test with.

[sbc100]

@dschuff or @aardappel could you take a quick look at this?

[dschuff]

This looks right to me too. But what are the provisional values used for in this case?

[sbc100]

In D96234#2549966, @dschuff wrote:

This looks right to me too. But what are the provisional values used for in this case?

Not really used for anything, but trigger warning in the linker if they are unexpected/wrong.

llvm/lib/MC/WasmObjectWriter.cpp

WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry,
   case wasm::R_WASM_MEMORY_ADDR_SLEB:
   case wasm::R_WASM_MEMORY_ADDR_SLEB64:
   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB64:
   case wasm::R_WASM_MEMORY_ADDR_I32:
   case wasm::R_WASM_MEMORY_ADDR_I64:
   case wasm::R_WASM_MEMORY_ADDR_TLS_SLEB: {
     // 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())
+    if (!RelEntry.Symbol->isDefined())
       return 0;
-    const wasm::WasmDataReference &BaseRef = DataLocations[Base],
-                                  &SymRef = DataLocations[RelEntry.Symbol];
-    const WasmDataSegment &Segment = DataSegments[BaseRef.Segment];
+    const wasm::WasmDataReference &SymRef = DataLocations[RelEntry.Symbol];
+    const WasmDataSegment &Segment = DataSegments[SymRef.Segment];
     // Ignore overflow. LLVM allows address arithmetic to silently wrap.
-    return Segment.Offset + BaseRef.Offset + SymRef.Offset + RelEntry.Addend;
+    return Segment.Offset + SymRef.Offset + RelEntry.Addend;
   }
   default:
     llvm_unreachable("invalid relocation type");
   }
 }
 
 static void addData(SmallVectorImpl<char> &DataBytes,
                     MCSectionWasm &DataSection) {

llvm/test/MC/WebAssembly/alias-offset.s

 main:
   .functype   main () -> ()
   i32.const sym_a
   i32.store sym_b
   end_function
 
 # CHECK-LABEL: <main>:
 # CHECK-EMPTY:
-# CHECK-NEXT:       3: 41 88 80 80 80 00     i32.const       8
+# CHECK-NEXT:       3: 41 84 80 80 80 00     i32.const       4
 # CHECK-NEXT:                        00000004:  R_WASM_MEMORY_ADDR_SLEB      sym_a+0
-# CHECK-NEXT:       9: 36 02 8c 80 80 80 00  i32.store       12
+# CHECK-NEXT:       9: 36 02 88 80 80 80 00  i32.store       8
 # CHECK-NEXT:                        0000000b:  R_WASM_MEMORY_ADDR_LEB      sym_b+0
 # CHECK-NEXT:      10: 0b            end