Partial support for common linkage for WebAssembly

Emulate common linkage through weak linkage, the only difference between
the two being mostly similar according to langref, with the notable
exception that common linkage implies zero initializer.

This is needed for fortran-to-web-assembly projects used in the python
community, esp.

https://github.com/emscripten-forge/recipes/tree/main/recipes/recipes_emscripten/libflang

Author: serge-sans-paille

llvm/lib/MC/MCWasmStreamer.cpp

@@ -14,6 +14,7 @@
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCCodeEmitter.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCObjectStreamer.h"
@@ -131,7 +132,29 @@ bool MCWasmStreamer::emitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
 
 void MCWasmStreamer::emitCommonSymbol(MCSymbol *S, uint64_t Size,
                                       Align ByteAlignment) {
-  llvm_unreachable("Common symbols are not yet implemented for Wasm");
+  auto *Symbol = cast<MCSymbolWasm>(S);
+
+  pushSection();
+
+  // Common symbols are very close to weak symbols, so manually build a weak
+  // symbol.
+  MCSectionWasm *SW = getContext().getWasmSection(".bss.common." + S->getName(),
+                                                  SectionKind::getData());
+  SW->setAlignment(ByteAlignment);
+  getAssembler().registerSection(*SW);
+
+  switchSection(SW);
+
+  MCFragment *F = getCurrentFragment();
+  F->appendContents(Size, '\0');
+  Symbol->setFragment(F);
+
+  Symbol->setSize(MCConstantExpr::create(Size, getContext(), false, 8));
+  Symbol->setWeak(true);
+  Symbol->setExternal(true);
+  getAssembler().registerSymbol(*Symbol);
+
+  popSection();
 }
 
 void MCWasmStreamer::emitELFSize(MCSymbol *Symbol, const MCExpr *Value) {

llvm/test/MC/WebAssembly/common.ll

@@ -0,0 +1,69 @@
+; RUN; llc -mcpu=mvp -filetype=obj %s -o - | obj2yaml | FileCheck %s
+; RUN; llc -mcpu=mvp -filetype=asm %s -asm-verbose=false -o -  | FileCheck --check-prefix=ASM %s
+; RUN: llc -mcpu=mvp -filetype=asm %s -o - | llvm-mc -triple=wasm32 -filetype=obj  -o - | obj2yaml | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+;target triple = "x86_64-redhat-linux-gnu"
+@b = common dso_local global [10 x i32] zeroinitializer, align 4
+@c = common dso_local global [20 x i32] zeroinitializer, align 32
+
+; CHECK-ASM: .file	"common.ll"
+; CHECK-ASM: .type	b,@object
+; CHECK-ASM: .comm	b,40,2
+; CHECK-ASM: .type	c,@object
+; CHECK-ASM: .comm	c,80,5
+
+
+; CHECK:      --- !WASM
+; CHECK-NEXT: FileHeader:
+; CHECK-NEXT:   Version:         0x1
+; CHECK-NEXT: Sections:
+; CHECK-NEXT:   - Type:            IMPORT
+; CHECK-NEXT:     Imports:
+; CHECK-NEXT:       - Module:          env
+; CHECK-NEXT:         Field:           __linear_memory
+; CHECK-NEXT:         Kind:            MEMORY
+; CHECK-NEXT:         Memory:
+; CHECK-NEXT:           Minimum:         0x1
+; CHECK-NEXT:   - Type:            DATACOUNT
+; CHECK-NEXT:     Count:           2
+; CHECK-NEXT:   - Type:            DATA
+; CHECK-NEXT:     Segments:
+; CHECK-NEXT:       - SectionOffset:   6
+; CHECK-NEXT:         InitFlags:       0
+; CHECK-NEXT:         Offset:
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           0
+; CHECK-NEXT:         Content:         '00000000000000000000000000000000000000000000000000000000000000000000000000000000'
+; CHECK-NEXT:       - SectionOffset:   52
+; CHECK-NEXT:         InitFlags:       0
+; CHECK-NEXT:         Offset:
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           64
+; CHECK-NEXT:         Content:         '0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'
+; CHECK-NEXT:   - Type:            CUSTOM
+; CHECK-NEXT:     Name:            linking
+; CHECK-NEXT:     Version:         2
+; CHECK-NEXT:     SymbolTable:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Kind:            DATA
+; CHECK-NEXT:         Name:            b
+; CHECK-NEXT:         Flags:           [ BINDING_WEAK ]
+; CHECK-NEXT:         Segment:         0
+; CHECK-NEXT:         Size:            40
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Kind:            DATA
+; CHECK-NEXT:         Name:            c
+; CHECK-NEXT:         Flags:           [ BINDING_WEAK ]
+; CHECK-NEXT:         Segment:         1
+; CHECK-NEXT:         Size:            80
+; CHECK-NEXT:     SegmentInfo:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Name:            .bss.common.b
+; CHECK-NEXT:         Alignment:       2
+; CHECK-NEXT:         Flags:           [  ]
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Name:            .bss.common.c
+; CHECK-NEXT:         Alignment:       5
+; CHECK-NEXT:         Flags:           [  ]
+; CHECK-NEXT: ...