[WebAssembly] Use data sections by default

[sbc100]

Summary

This allows data sections that don't start with .data to be
used/created.

Without this, clang's __attribute__((section("foo"))) would
generate assembly that would not parse.

[dschuff]

So the issue here is that using attribute(section) just generates the equivalent of .section and expects it to be a data section by default? Can you use attribute(section) on a function too? what's the behavior?

[sbc100]

In D96233#2551960, @dschuff wrote:

So the issue here is that using attribute(section) just generates the equivalent of .section and expects it to be a data section by default? Can you use attribute(section) on a function too? what's the behavior?

No, with wasm we don't support the section attribute for functions. Functions always get a unique section of their own. Also, unlike data sections you cannot get the start_xxx and end_xxx of a text section or find any addresses within a text section so it makes almost no sense to try to do this with wem.

llvm/lib/MC/MCParser/WasmAsmParser.cpp

auto Kind = StringSwitch<Optional<SectionKind>>(Name)
                     .StartsWith(".rodata", SectionKind::getReadOnly())
                     .StartsWith(".text", SectionKind::getText())
                     .StartsWith(".custom_section", SectionKind::getMetadata())
                     .StartsWith(".bss", SectionKind::getBSS())
                     // See use of .init_array in WasmObjectWriter and
                     // TargetLoweringObjectFileWasm
                     .StartsWith(".init_array", SectionKind::getData())
                     .StartsWith(".debug_", SectionKind::getMetadata())
-                    .Default(Optional<SectionKind>());
-    if (!Kind.hasValue())
-      return Parser->Error(Lexer->getLoc(), "unknown section kind: " + Name);
-
+                    .Default(SectionKind::getData());
 
     // Update section flags if present in this .section directive
     bool Passive = false;
     bool Group = false;
     if (parseSectionFlags(getTok().getStringContents(), Passive, Group))
       return true;
 
     Lex();

llvm/test/MC/WebAssembly/data-section-combined.s

+# Test that placing multiple data symbols in the same section works
+
+# RUN: llvm-mc -triple=wasm32-unknown-unknown < %s | FileCheck %s
+
+test0:
+    .functype   test0 () -> (i32)
+    i32.const a
+    i32.const b
+    end_function
+
+    .section mysec,"",@
+a:
+    .int32 42
+    .int32 43
+    .size a, 8
+b:
+    .int32 44
+    .size b, 4
+
+#      CHECK:   .section  mysec,"",@
+# CHECK-NEXT: a:
+# CHECK-NEXT:   .int32 42
+# CHECK-NEXT:   .int32 43
+# CHECK-NEXT:   .size a, 8
+# CHECK-NEXT: b:
+# CHECK-NEXT:   .int32 44
+# CHECK-NEXT:   .size b, 4
+
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown < %s | llvm-objdump --triple=wasm32-unknown-unknown -d -t -r - | FileCheck %s --check-prefix=OBJ
+
+
+#      OBJ: 00000001 <test0>:
+#      OBJ:        3: 41 80 80 80 80 00     i32.const       0
+# OBJ-NEXT:                         00000004:  R_WASM_MEMORY_ADDR_SLEB      a+0
+# OBJ-NEXT:        9: 41 88 80 80 80 00     i32.const       8
+# OBJ-NEXT:                         0000000a:  R_WASM_MEMORY_ADDR_SLEB      b+0
+# OBJ-NEXT:        f: 0b            end