[WebAssembly] MC: Start table at offset 1 rather than 0

[sbc100]

Summary

For consistency with the output of lld.

This is useful in runnable binaries as can them be sure the
null function pointer will never be a valid argument
call_indirect.

[ncw]

I can't see that anything would break by landing this and D42285 first.

There are some other changes in D42095 as well, because even with this offset-by-one change, the indexes written out by Clang still don't match the indexes LLD would write out for the same Symbols (for weak and aliased functions, of course!). But, the rest of #3 (D42095) definitely can't land before #2. Until the actual symbol indexes are being written out for the table relocations (#2), we can't alter the ELEM section to use exactly the same rules in Clang as it does in LLD.

[sbc100]

I was trying to tease out this part that could land without and ABI change (that TABLE_INDEX relocation change). Hopefully we can do the ABI changes this week too though.

lib/MC/WasmObjectWriter.cpp

 #include <vector>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "mc"
 
 namespace {
 
+// Went we ceate the indirect function table we start at 1, so that there is
+// and emtpy slot at 0 and therefore calling a null function pointer will trap.
+static const uint32_t kInitialTableOffset = 1;
+
 // For patching purposes, we need to remember where each section starts, both
 // for patching up the section size field, and for patching up references to
 // locations within the section.
 struct SectionBookkeeping {
   // Where the size of the section is written.
   uint64_t SizeOffset;
   // Where the contents of the section starts (after the header).
   uint64_t ContentsOffset;
void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
   SectionBookkeeping Section;
   startSection(Section, wasm::WASM_SEC_ELEM);
 
   encodeULEB128(1, getStream()); // number of "segments"
   encodeULEB128(0, getStream()); // the table index
 
   // init expr for starting offset
   write8(wasm::WASM_OPCODE_I32_CONST);
-  encodeSLEB128(0, getStream());
+  encodeSLEB128(kInitialTableOffset, getStream());
   write8(wasm::WASM_OPCODE_END);
 
   encodeULEB128(TableElems.size(), getStream());
   for (uint32_t Elem : TableElems)
     encodeULEB128(Elem, getStream());
 
   endSection(Section);
 }
auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
       const MCSymbolWasm& WS = *Rel.Symbol;
       if (WS.isFunction() && IndirectSymbolIndices.count(&WS) == 0) {
         switch (Rel.Type) {
         case wasm::R_WEBASSEMBLY_TABLE_INDEX_I32:
         case wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB:
         case wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32:
         case wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB: {
           uint32_t Index = SymbolIndices.find(&WS)->second;
-          IndirectSymbolIndices[&WS] = TableElems.size();
+          IndirectSymbolIndices[&WS] = TableElems.size() + kInitialTableOffset;
           DEBUG(dbgs() << "  -> adding to table: " << TableElems.size() << "\n");
           TableElems.push_back(Index);
           registerFunctionType(WS);
           break;
         }
         default:
           break;
         }

test/MC/WebAssembly/external-func-address.ll

 ; CHECK:            - Module:          env
 ; CHECK-NEXT:         Field:           f1
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         SigIndex:        0
 ; CHECK:        - Type:            ELEM
 ; CHECK-NEXT:     Segments:
 ; CHECK-NEXT:       - Offset:
 ; CHECK-NEXT:           Opcode:          I32_CONST
-; CHECK-NEXT:           Value:           0
+; CHECK-NEXT:           Value:           1
 ; CHECK-NEXT:         Functions:       [ 0 ]
 ; CHECK:        - Type:            DATA
 ; CHECK-NEXT:     Relocations:
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_TABLE_INDEX_I32
-; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x00000006

test/MC/WebAssembly/func-address.ll

 ; CHECK:     Relocation {
 ; CHECK:       Type: R_WEBASSEMBLY_FUNCTION_INDEX_LEB (0)
 ; CHECK:       Offset: 0xB
 ; CHECK:       Index: 0x1
 ; CHECK:     }
 ; CHECK:     Relocation {
 ; CHECK:       Type: R_WEBASSEMBLY_TABLE_INDEX_SLEB (1)
 ; CHECK:       Offset: 0x1E
-; CHECK:       Index: 0x0
+; CHECK:       Index: 0x1
 ; CHECK:     }
 ; CHECK:   }

test/MC/WebAssembly/global-ctor-dtor.ll

 ; CHECK-NEXT:         Index:           8
 ; CHECK-NEXT:       - Name:            global1
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
-; CHECK-NEXT:           Value:           0
+; CHECK-NEXT:           Value:           1
 ; CHECK-NEXT:         Functions:       [ 5, 7 ]
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Relocations:     
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:         Offset:          0x00000004
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_TABLE_INDEX_SLEB
-; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x0000000F
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_MEMORY_ADDR_SLEB
 ; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:         Offset:          0x00000017
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x0000001D
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           2
 ; CHECK-NEXT:         Offset:          0x0000002C
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_TABLE_INDEX_SLEB
-; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Index:           2
 ; CHECK-NEXT:         Offset:          0x00000037
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_MEMORY_ADDR_SLEB
 ; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:         Offset:          0x0000003F
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x00000045
 ; CHECK-NEXT:     Functions:       
 ; CHECK-NEXT:       - Index:           5
 ; CHECK-NEXT:         Locals:          
 ; CHECK-NEXT:         Body:            1080808080000B
 ; CHECK-NEXT:       - Index:           6
 ; CHECK-NEXT:         Locals:          
-; CHECK-NEXT:         Body:            024041808080800041004180808080001081808080000D000F0B00000B
+; CHECK-NEXT:         Body:            024041818080800041004180808080001081808080000D000F0B00000B
 ; CHECK-NEXT:       - Index:           7
 ; CHECK-NEXT:         Locals:          
 ; CHECK-NEXT:         Body:            1082808080000B
 ; CHECK-NEXT:       - Index:           8
 ; CHECK-NEXT:         Locals:          
-; CHECK-NEXT:         Body:            024041818080800041004180808080001081808080000D000F0B00000B
+; CHECK-NEXT:         Body:            024041828080800041004180808080001081808080000D000F0B00000B
 ; CHECK-NEXT:   - Type:            DATA
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - SectionOffset:   6
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           0
 ; CHECK-NEXT:         Content:         '01040000'

test/MC/WebAssembly/weak-alias.ll

 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:       - Name:            bar_alias
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         Index:           3
 ; CHECK-NEXT:   - Type:            ELEM
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
-; CHECK-NEXT:           Value:           0
+; CHECK-NEXT:           Value:           1
 ; CHECK-NEXT:         Functions:       [ 1, 0 ]
 ; CHECK-NEXT:   - Type:            CODE
 ; CHECK-NEXT:     Relocations:     
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x00000009
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           0

 ; CHECK-NEXT:         Locals:          
 ; CHECK-NEXT:         Body:            410028028880808000118080808000000B
 ; CHECK-NEXT:       - Index:           5
 ; CHECK-NEXT:         Locals:          
 ; CHECK-NEXT:         Body:            410028029080808000118080808000000B
 ; CHECK-NEXT:   - Type:            DATA
 ; CHECK-NEXT:     Relocations:     
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_TABLE_INDEX_I32
-; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x0000000F
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_TABLE_INDEX_I32
-; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Index:           2
 ; CHECK-NEXT:         Offset:          0x00000018
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - SectionOffset:   6
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           0
 ; CHECK-NEXT:         Content:         '07000000'
 ; CHECK-NEXT:       - SectionOffset:   15
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           8
-; CHECK-NEXT:         Content:         '00000000'
+; CHECK-NEXT:         Content:         '01000000'
 
 ; CHECK:        - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            name
 ; CHECK-NEXT:     FunctionNames:   
 ; CHECK-NEXT:       - Index:           0
 ; CHECK-NEXT:         Name:            foo_alias
 ; CHECK-NEXT:       - Index:           1
 ; CHECK-NEXT:         Name:            foo