[wasm-ld][Coverage] Align __llvm_{covfun,covmap} sections to 8 bytes

This patch aligns the `__llvm_{covfun,covmap}` sections to 8 bytes at
the linking stage. This is required because llvm-cov assumes that
records in the sections are 8-byte aligned. Unfortunately, custom
sections does not have an alignment attribute in the WebAssembly linking
spec, so we temporarily do a special case for the metadata sections.
In the future, we should generalize this to support alignment for
relocatable custom sections.

Author: kateinoigakukun

lld/test/wasm/custom-section-align.s

@@ -0,0 +1,31 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld --no-entry %t.o -o %t.wasm
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+# Check that "__llvm_covfun" custom section is aligned to 8 bytes.
+
+        .section        .custom_section.__llvm_covfun,"GR",@,__covrec_A
+        .int32  1
+        .int8   2
+# pad   .int8   0
+#       .int8   0
+#       .int8   0
+
+        .section        .custom_section.__llvm_covfun,"GR",@,__covrec_B
+        .int32  3
+
+# CHECK:      - Type:            CUSTOM
+# CHECK-NEXT:   Name:            __llvm_covfun
+# CHECK-NEXT:   Payload:         '010000000200000003000000'
+
+# Check that regular custom sections are not aligned.
+        .section        .custom_section.foo,"GR",@,foo_A
+        .int32  1
+        .int8   2
+
+        .section        .custom_section.foo,"GR",@,foo_B
+        .int32  3
+
+# CHECK:      - Type:            CUSTOM
+# CHECK-NEXT:   Name:            foo
+# CHECK-NEXT:   Payload:         '010000000203000000'

lld/wasm/InputChunks.h

@@ -177,8 +177,9 @@ class MergeInputChunk : public InputChunk {
     inputSectionOffset = seg.SectionOffset;
   }
 
-  MergeInputChunk(const WasmSection &s, ObjFile *f)
-      : InputChunk(f, Merge, s.Name, 0, llvm::wasm::WASM_SEG_FLAG_STRINGS) {
+  MergeInputChunk(const WasmSection &s, ObjFile *f, uint32_t alignment)
+      : InputChunk(f, Merge, s.Name, alignment,
+                   llvm::wasm::WASM_SEG_FLAG_STRINGS) {
     assert(s.Type == llvm::wasm::WASM_SEC_CUSTOM);
     comdat = s.Comdat;
     rawData = s.Content;
@@ -234,6 +235,7 @@ class SyntheticMergedChunk : public InputChunk {
 
   void addMergeChunk(MergeInputChunk *ms) {
     comdat = ms->getComdat();
+    alignment = std::max(alignment, ms->alignment);
     ms->parent = this;
     chunks.push_back(ms);
   }
@@ -337,8 +339,8 @@ class SyntheticFunction : public InputFunction {
 // Represents a single Wasm Section within an input file.
 class InputSection : public InputChunk {
 public:
-  InputSection(const WasmSection &s, ObjFile *f)
-      : InputChunk(f, InputChunk::Section, s.Name),
+  InputSection(const WasmSection &s, ObjFile *f, uint32_t alignment)
+      : InputChunk(f, InputChunk::Section, s.Name, alignment),
         tombstoneValue(getTombstoneForSection(s.Name)), section(s) {
     assert(section.Type == llvm::wasm::WASM_SEC_CUSTOM);
     comdat = section.Comdat;

lld/wasm/InputFiles.cpp

@@ -18,6 +18,7 @@
 #include "llvm/BinaryFormat/Wasm.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Object/Wasm.h"
+#include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/TarWriter.h"
 #include "llvm/Support/raw_ostream.h"
@@ -451,6 +452,18 @@ void SharedFile::parse() {
   }
 }
 
+/// Returns the alignment for a custom section. This is used to concatenate
+/// custom sections with the same name into a single custom section.
+static uint32_t getCustomSectionAlignment(const WasmSection &sec) {
+  // TODO: Add a section attribute for alignment in the linking spec.
+  if (sec.Name == getInstrProfSectionName(IPSK_covfun, Triple::Wasm) ||
+      sec.Name == getInstrProfSectionName(IPSK_covmap, Triple::Wasm)) {
+    // llvm-cov assumes that coverage metadata sections are 8-byte aligned.
+    return 8;
+  }
+  return 1;
+}
+
 WasmFileBase::WasmFileBase(Kind k, MemoryBufferRef m) : InputFile(k, m) {
   // Parse a memory buffer as a wasm file.
   LLVM_DEBUG(dbgs() << "Reading object: " << toString(this) << "\n");
@@ -520,10 +533,11 @@ void ObjFile::parse(bool ignoreComdats) {
       dataSection = &section;
     } else if (section.Type == WASM_SEC_CUSTOM) {
       InputChunk *customSec;
+      uint32_t alignment = getCustomSectionAlignment(section);
       if (shouldMerge(section))
-        customSec = make<MergeInputChunk>(section, this);
+        customSec = make<MergeInputChunk>(section, this, alignment);
       else
-        customSec = make<InputSection>(section, this);
+        customSec = make<InputSection>(section, this, alignment);
       customSec->discarded = isExcludedByComdat(customSec);
       customSections.emplace_back(customSec);
       customSections.back()->setRelocations(section.Relocations);

lld/wasm/OutputSections.cpp

@@ -244,6 +244,7 @@ void CustomSection::finalizeContents() {
 
   for (InputChunk *section : inputSections) {
     assert(!section->discarded);
+    payloadSize = alignTo(payloadSize, section->alignment);
     section->outSecOff = payloadSize;
     payloadSize += section->getSize();
   }