[LLD] [COFF] Set proper pointer size alignment for LocalImportChunk

[mstorsjo]

Summary

When these are accessed with load/store instructions on ARM64, it becomes strictly necessary to have them properly aligned.

This fixes PR39228.

This is, IMO, a good candidate for backporting to the 7.0.1 branch.

[ruiu]

LGTM

COFF/Chunks.h

372

I'd write Alignment = Config->Is64 ? 8 : 4; instead of getSize() because it can be understood without reading getSize(). (I wondered whether or not getSize() returns a large value such as 16 when I read this code for the first time.)

COFF/Chunks.h

public:
 
   Defined *Target;
 };
 
 // Windows-specific.
 // See comments for DefinedLocalImport class.
 class LocalImportChunk : public Chunk {
 public:
-  explicit LocalImportChunk(Defined *S) : Sym(S) {}
+  explicit LocalImportChunk(Defined *S) : Sym(S) {
+    Alignment = getSize();
+  }
   size_t getSize() const override;
   void getBaserels(std::vector<Baserel> *Res) override;
   void writeTo(uint8_t *Buf) const override;
 
 private:
   Defined *Sym;
 };
 

test/COFF/arm64-localimport-align.s

+// REQUIRES: aarch64
+// RUN: llvm-mc -filetype=obj -triple=aarch64-windows %s -o %t.obj
+// RUN: lld-link -entry:main -subsystem:console %t.obj -out:%t.exe
+// Don't check the output, just make sure it links fine and doesn't
+// error out due to a misaligned load.
+    .text
+    .globl main
+    .globl myfunc
+main:
+    adrp x8, __imp_myfunc
+    ldr  x0, [x8, :lo12:__imp_myfunc]
+    br   x0
+    ret
+myfunc:
+    ret
+
+    .section .rdata, "dr"
+    // Start the .rdata section with a 4 byte chunk, to expose the alignment
+    // of the next chunk in the section.
+mydata:
+    .byte 42
+    // The synthesized LocalImportChunk gets stored here in the .rdata
+    // section, but needs to get proper 8 byte alignment since it is a
+    // pointer, just like regular LookupChunks in the IAT.