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

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.

Subscribers: jfb, dschuff, jgravelle-google, aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D42284

llvm-svn: 322978

5 files changed, 18 insertions, 14 deletions

diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 0fe04f188d7..473f9fe7ede 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -38,6 +38,10 @@ using namespace llvm;
 
 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.
@@ -789,7 +793,7 @@ void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
 
   // 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());
@@ -1326,7 +1330,7 @@ void WasmObjectWriter::writeObject(MCAssembler &Asm,
         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);
diff --git a/llvm/test/MC/WebAssembly/external-func-address.ll b/llvm/test/MC/WebAssembly/external-func-address.ll
index 9386d945ccd..c8dff4036d3 100644
--- a/llvm/test/MC/WebAssembly/external-func-address.ll
+++ b/llvm/test/MC/WebAssembly/external-func-address.ll
@@ -29,10 +29,10 @@ declare void @f1(i32) #1
 ; 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
diff --git a/llvm/test/MC/WebAssembly/func-address.ll b/llvm/test/MC/WebAssembly/func-address.ll
index 16d38e1ebf2..8532aa9a9c7 100644
--- a/llvm/test/MC/WebAssembly/func-address.ll
+++ b/llvm/test/MC/WebAssembly/func-address.ll
@@ -42,6 +42,6 @@ entry:
 ; CHECK:     Relocation {
 ; CHECK:       Type: R_WEBASSEMBLY_TABLE_INDEX_SLEB (1)
 ; CHECK:       Offset: 0x1E
-; CHECK:       Index: 0x0
+; CHECK:       Index: 0x1
 ; CHECK:     }
 ; CHECK:   }
diff --git a/llvm/test/MC/WebAssembly/global-ctor-dtor.ll b/llvm/test/MC/WebAssembly/global-ctor-dtor.ll
index 759c72772bc..75f00445691 100644
--- a/llvm/test/MC/WebAssembly/global-ctor-dtor.ll
+++ b/llvm/test/MC/WebAssembly/global-ctor-dtor.ll
@@ -87,7 +87,7 @@ declare void @func3()
 ; 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:     
@@ -95,7 +95,7 @@ declare void @func3()
 ; 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
@@ -107,7 +107,7 @@ declare void @func3()
 ; 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
@@ -121,13 +121,13 @@ declare void @func3()
 ; 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
diff --git a/llvm/test/MC/WebAssembly/weak-alias.ll b/llvm/test/MC/WebAssembly/weak-alias.ll
index 1698913564f..83f2eebbe37 100644
--- a/llvm/test/MC/WebAssembly/weak-alias.ll
+++ b/llvm/test/MC/WebAssembly/weak-alias.ll
@@ -130,7 +130,7 @@ entry:
 ; 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:     
@@ -171,10 +171,10 @@ entry:
 ; 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
@@ -188,7 +188,7 @@ entry:
 ; 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