[ELF] Simplify handling of AT section attribute.

[ikudrin]

Summary

The patch is going to replace the implementation from r323625.

The original implementation adds a section with AT attribute into the same load segment
as a previous section if that segment does not contain a section with a customized LMA.
After that, this attribute affects not only this and following sections, as required by specs,
but also previous sections in that segment.

I believe that a better way is to just start a new segment if we encounter a section
with AT attribute, with the only exception for the very first section, because we have added
header sections in the first load segment. It looks like GNU ld does the same.

[grimar]

test/ELF/linkerscript/at5.s

10 ↗

(On Diff #134198)

Seems you forget to remove debug tee %t2.log

10 ↗

(On Diff #134198)

*forgot

[ikudrin]

• Remove "tee". Thanks, George!

[ikudrin]

Update according to Rafael's comments:

• Replace !FirstLoadSection with Load->LastSec != Out::ProgramHeaders.
• Add a corresponding comment.

[ikudrin]

Accepted by @rafael in the mailing list.

lld/trunk/ELF/Writer.h

struct PhdrEntry {
   uint32_t p_align = 0;
   uint32_t p_type = 0;
   uint32_t p_flags = 0;
 
   OutputSection *FirstSec = nullptr;
   OutputSection *LastSec = nullptr;
   bool HasLMA = false;
 
-  // True if one of the sections in this program header has a LMA specified via
-  // linker script: AT(addr). We never allow 2 or more sections with LMA in the
-  // same program header.
-  bool ASectionHasLMA = false;
-
   uint64_t LMAOffset = 0;
 };
 
 void addReservedSymbols();
 llvm::StringRef getOutputSectionName(InputSectionBase *S);
 
 template <class ELFT> uint32_t calcMipsEFlags();
 

lld/trunk/ELF/Writer.cpp

 
 void PhdrEntry::add(OutputSection *Sec) {
   LastSec = Sec;
   if (!FirstSec)
     FirstSec = Sec;
   p_align = std::max(p_align, Sec->Alignment);
   if (p_type == PT_LOAD)
     Sec->PtLoad = this;
-  if (Sec->LMAExpr)
-    ASectionHasLMA = true;
 }
 
 // The beginning and the ending of .rel[a].plt section are marked
 // with __rel[a]_iplt_{start,end} symbols if it is a statically linked
 // executable. The runtime needs these symbols in order to resolve
 // all IRELATIVE relocs on startup. For dynamic executables, we don't
 // need these symbols, since IRELATIVE relocs are resolved through GOT
 // and PLT. For details, see http://www.airs.com/blog/archives/403.
if (!(Sec->Flags & SHF_ALLOC))
       break;
     if (!needsPtLoad(Sec))
       continue;
 
     // Segments are contiguous memory regions that has the same attributes
     // (e.g. executable or writable). There is one phdr for each segment.
     // Therefore, we need to create a new phdr when the next section has
     // different flags or is loaded at a discontiguous address using AT linker
-    // script command.
+    // script command. At the same time, we don't want to create a separate
+    // load segment for the headers, even if the first output section has
+    // an AT attribute.
     uint64_t NewFlags = computeFlags(Sec->getPhdrFlags());
-    if ((Sec->LMAExpr && Load->ASectionHasLMA) ||
+    if ((Sec->LMAExpr && Load->LastSec != Out::ProgramHeaders) ||
         Sec->MemRegion != Load->FirstSec->MemRegion || Flags != NewFlags) {
 
       Load = AddHdr(PT_LOAD, NewFlags);
       Flags = NewFlags;
     }
 
     Load->add(Sec);
   }

lld/trunk/test/ELF/linkerscript/at4.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { \
+# RUN:  . = 0x1000; \
+# RUN:  .aaa : { *(.aaa) } \
+# RUN:  .bbb : AT(0x2008) { *(.bbb) } \
+# RUN:  .ccc : { *(.ccc) } \
+# RUN: }" > %t.script
+# RUN: ld.lld %t --script %t.script -o %t2
+# RUN: llvm-readobj -program-headers %t2 | FileCheck %s
+
+# CHECK:        Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x1000
+# CHECK-NEXT:     VirtualAddress: 0x1000
+# CHECK-NEXT:     PhysicalAddress: 0x1000
+# CHECK-NEXT:     FileSize: 8
+# CHECK-NEXT:     MemSize: 8
+# CHECK:        Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x1008
+# CHECK-NEXT:     VirtualAddress: 0x1008
+# CHECK-NEXT:     PhysicalAddress: 0x2008
+# CHECK-NEXT:     FileSize: 17
+# CHECK-NEXT:     MemSize: 17
+
+.global _start
+_start:
+ nop
+
+.section .aaa, "a"
+.quad 0
+
+.section .bbb, "a"
+.quad 0
+
+.section .ccc, "a"
+.quad 0