[yaml2obj] Simplify p_filesz/p_memsz computing

[MaskRay]

Summary

This fixes a bug as well. When "FileSize:" (p_filesz) is specified and
different from the actual value, the following code probably should not
use PHeader.p_filesz:

if (SHeader->sh_offset == PHeader.p_offset + PHeader.p_filesz)
  PHeader.p_memsz += SHeader->sh_size;

[grimar]

This looks good. Would be nice if someone else also take a look.

lib/ObjectYAML/ELFEmitter.cpp

630

When I first saw FileEnd I thought it is somehow related to the end of file.
Maybe FileOffset vs MemOffset is better? (Or some different better names)

[MaskRay]

Support multiple trailing SHT_NOBITS.

lld/ELF/Writer.cpp does not use the trick (two or more SHT_NOBITS are tricky and IIRC, when more than 1 SHF_TLS SHT_NOBITS sections are used, ld.bfd can get addresses wrong). However, since I can do it with 1 fewer line here :) Let me do it.

[MaskRay]

FileEnd -> FileOffset
MemEnd -> MemOffset

[jhenderson]

LGTM, I think.

lib/ObjectYAML/ELFEmitter.cpp

if (YamlPhdr.Offset) {
       else
         PHeader.p_offset = 0;
 
       // Find the minimum offset for the program header.
       for (Elf_Shdr *SHeader : Sections)
         PHeader.p_offset = std::min(PHeader.p_offset, SHeader->sh_offset);
     }
 
-    // Find the maximum offset of the end of a section in order to set p_filesz,
-    // if not set explicitly.
-    if (YamlPhdr.FileSize) {
-      PHeader.p_filesz = *YamlPhdr.FileSize;
-    } else {
-      PHeader.p_filesz = 0;
-      for (Elf_Shdr *SHeader : Sections) {
-        uint64_t EndOfSection;
-        if (SHeader->sh_type == llvm::ELF::SHT_NOBITS)
-          EndOfSection = SHeader->sh_offset;
-        else
-          EndOfSection = SHeader->sh_offset + SHeader->sh_size;
-        uint64_t EndOfSegment = PHeader.p_offset + PHeader.p_filesz;
-        EndOfSegment = std::max(EndOfSegment, EndOfSection);
-        PHeader.p_filesz = EndOfSegment - PHeader.p_offset;
-      }
+    // Find the maximum offset of the end of a section in order to set p_filesz
+    // and p_memsz. When setting p_filesz, sh_size is seen as 0 for SHT_NOBITS
+    // sections.
+    uint64_t FileEnd = PHeader.p_offset, MemEnd = PHeader.p_offset;
+      uint64_t End = SHeader->sh_offset + SHeader->sh_size;
+      MemEnd = std::max(MemEnd, End);
+
+        End = SHeader->sh_offset;
+      FileEnd = std::max(FileEnd, End);
     }
 
-    // If not set explicitly, find the memory size by adding the size of
-    // sections at the end of the segment. These should be empty (size of zero)
-    // and NOBITS sections.
-    if (YamlPhdr.MemSize) {
-      PHeader.p_memsz = *YamlPhdr.MemSize;
-    } else {
-      PHeader.p_memsz = PHeader.p_filesz;
-      for (Elf_Shdr *SHeader : Sections)
-        if (SHeader->sh_offset == PHeader.p_offset + PHeader.p_filesz)
-          PHeader.p_memsz += SHeader->sh_size;
-    }
+    // Set the file size and the memory size if not set explicitly.
+    PHeader.p_filesz = YamlPhdr.FileSize ? uint64_t(*YamlPhdr.FileSize)
+                                         : FileEnd - PHeader.p_offset;
+    PHeader.p_memsz = YamlPhdr.MemSize ? uint64_t(*YamlPhdr.MemSize)
+                                       : MemEnd - PHeader.p_offset;
 
     // Set the alignment of the segment to be the same as the maximum alignment
     // of the sections with the same offset so that by default the segment
     // has a valid and sensible alignment.
     if (YamlPhdr.Align) {
       PHeader.p_align = *YamlPhdr.Align;
     } else {
       PHeader.p_align = 1;

test/tools/yaml2obj/program-header-size-offset.yaml

 # Show that yaml2obj properly emits program headers with explicit file size,
 # memory size and offset parameters.
 
 # RUN: yaml2obj %s -o %t
 # RUN: llvm-readobj %t --program-headers | FileCheck %s
 
 # CHECK: ProgramHeaders [
 # CHECK:    Offset: 0x1234
 # CHECK:    FileSize: 1111
 # CHECK:    MemSize: 9999
 
 # CHECK:    Offset: 0x2000
 # CHECK:    FileSize: 6
-# CHECK:    MemSize: 6
+# CHECK:    MemSize: 4
 
 # CHECK:    Offset: 0x2000
 # CHECK:    FileSize: 4
 # CHECK:    MemSize: 6
 
 # CHECK:    Offset: 0x1FFF
 # CHECK:    FileSize: 5
 # CHECK:    MemSize: 5