Object: The default alignment of a section without alignment flags is 16.

[pcc]

Summary

Depends on D46419

[hans]

Though I wish it were true, I'm not convinced yet that this is what link.exe does; see my comment at https://crbug.com/837667#c24

[hans]

In D46420#1087518, @hans wrote:

Though I wish it were true, I'm not convinced yet that this is what link.exe does; see my comment at https://crbug.com/837667#c24

Okay, I convinced myself in https://crbug.com/837667#c27 :-)

lld/test/COFF/default-alignment.test

+# RUN: yaml2obj < %s > %t.obj
+# RUN: lld-link /out:%t.exe %t.obj /entry:__ImageBase /subsystem:console
+# RUN: llvm-objdump -s %t.exe | FileCheck %s
+
+# CHECK: Contents of section .rdata:
+# CHECK-NEXT: 01000000 00000000 00000000 00000000
+# CHECK-NEXT: 02
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .rdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    SectionData:     01
+  - Name:            .rdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    SectionData:     02
+symbols:
+...

llvm/include/llvm/Object/COFF.h

struct coff_section {
   }
 
   uint32_t getAlignment() const {
     // The IMAGE_SCN_TYPE_NO_PAD bit is a legacy way of getting to
     // IMAGE_SCN_ALIGN_1BYTES.
     if (Characteristics & COFF::IMAGE_SCN_TYPE_NO_PAD)
       return 1;
 
-    // Bit [20:24] contains section alignment. Both 0 and 1 mean alignment 1.
+    // Bit [20:24] contains section alignment. 0 means use a default alignment
+    // of 16.
     uint32_t Shift = (Characteristics >> 20) & 0xF;
     if (Shift > 0)
       return 1U << (Shift - 1);
-    return 1;
+    return 16;
   }
 };
 
 struct coff_relocation {
   support::ulittle32_t VirtualAddress;
   support::ulittle32_t SymbolTableIndex;
   support::ulittle16_t Type;
 };