Align output segments correctly

Previously, segments were aligned according to their first section's
alignment requirements. That was not correct, but segments are also
aligned to a page boundary, and a page boundary is usually much larger
than a section alignment requirement, so no one noticed this bug before.

Now, lld has --nmagic option which sets maxPageSize to 1 to effectively
disable page alignment, which reveals the issue.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43212

Differential Revision: https://reviews.llvm.org/D67152

llvm-svn: 371013

lld/ELF/Writer.cpp

@@ -2272,7 +2272,8 @@ static uint64_t computeFileOffset(OutputSection *os, uint64_t off) {
   // The first section in a PT_LOAD has to have congruent offset and address
   // module the page size.
   if (os->ptLoad && os->ptLoad->firstSec == os) {
-    uint64_t alignment = std::max<uint64_t>(os->alignment, config->maxPageSize);
+    uint64_t alignment =
+        std::max<uint64_t>(os->ptLoad->p_align, config->maxPageSize);
     return alignTo(off, alignment, os->addr);
   }
 

lld/test/ELF/nmagic.s

@@ -0,0 +1,23 @@
+# REQUIRES: x86
+# Verify that .rodata is aligned to a 8 byte boundary.
+
+# RUN: llvm-mc -filetype=obj -triple=i386 %s -o %t.o
+# RUN: ld.lld %t.o -o %t.exe -n -Ttext 0
+# RUN: llvm-readelf --section-headers %t.exe | FileCheck %s
+
+# CHECK: [ 0]           NULL     00000000 000000 000000 00      0   0  0
+# CHECK: [ 1] .text     PROGBITS 00000000 0000d4 000001 00  AX  0   0  4
+# CHECK: [ 2] .rodata   PROGBITS 00000008 0000d8 000008 00   A  0   0  8
+# CHECK: [ 3] .comment  PROGBITS 00000000 0000e0 000008 01  MS  0   0  1
+# CHECK: [ 4] .symtab   SYMTAB   00000000 0000e8 000020 10      6   1  4
+# CHECK: [ 5] .shstrtab STRTAB   00000000 000108 000032 00      0   0  1
+# CHECK: [ 6] .strtab   STRTAB   00000000 00013a 000008 00      0   0  1
+
+.globl _start
+.text
+_start:
+  ret
+
+.rodata
+.align 8
+.quad 42