[ELF] - Postpone evaluation of LMA offset.

[grimar]

Summary

Previously we evaluated the values of LMA incorrectly for next cases:

.text : AT(ADDR(.text) - 0xffffffff80000000) { ... }
.data : AT(ADDR(.data) - 0xffffffff80000000) { .. }
.init.begin : AT(ADDR(.init.begin) - 0xffffffff80000000) { ... }

Reason was that we evaluated offset when VA was not assigned. For case above
we ended up with 3 loads that has similar LMA and it was incorrect.
That is critical for linux kernel.

Patch updates the offset after VA calculation. That fixes the issue I observing.

[ruiu]

lld/trunk/ELF/LinkerScript.cpp

568 ↗

(On Diff #89209)

If you change this to

LMAOffset = [=] { return Cmd->LMAExpr(Dot) - Dot; }

I generally don't like to overuse std::pair because its member names (first and second) sometimes hurts code readability.

lld/trunk/ELF/LinkerScript.h

301 ↗

(On Diff #89209)

This can be just Expr LMAOffset, right?

[grimar]

lld/trunk/ELF/LinkerScript.cpp

568 ↗

(On Diff #89209)

That would work if I do something like:

if (Cmd->LMAExpr) {
  uintX_t D = Dot;
  LMAOffset = [=] { return Cmd->LMAExpr(D) - D; };
}

Because you can't capture member Dot by value.
Does it look fine for you ?

lld/trunk/ELF/LinkerScript.h

301 ↗

(On Diff #89209)

I would use std::function<uint64_t()> then. Expr assumes Dot argument which is unused.
And would complicate the initialization to:

LMAOffset = {[=](uint64_t) { return Cmd->LMAExpr(D) - D; }};

ELF/LinkerScript.h

private:
 
   std::vector<size_t> getPhdrIndices(StringRef SectionName);
   size_t getPhdrIndex(const Twine &Loc, StringRef PhdrName);
 
   MemoryRegion *findMemoryRegion(OutputSectionCommand *Cmd,
                                  OutputSectionBase *Sec);
 
   uintX_t Dot;
-  uintX_t LMAOffset = 0;
+  std::pair<Expr, uintX_t> CurLMA;
   OutputSectionBase *CurOutSec = nullptr;
   MemoryRegion *CurMemRegion = nullptr;
   uintX_t ThreadBssOffset = 0;
   void switchTo(OutputSectionBase *Sec);
   void flush();
   void output(InputSection<ELFT> *Sec);
   void process(BaseCommand &Base);
   llvm::DenseSet<OutputSectionBase *> AlreadyOutputOS;

ELF/LinkerScript.cpp

void LinkerScript<ELFT>::switchTo(OutputSectionBase *Sec) {
 
   Dot = alignTo(Dot, CurOutSec->Addralign);
   CurOutSec->Addr = isTbss<ELFT>(CurOutSec) ? Dot + ThreadBssOffset : Dot;
 
   // If neither AT nor AT> is specified for an allocatable section, the linker
   // will set the LMA such that the difference between VMA and LMA for the
   // section is the same as the preceding output section in the same region
   // https://sourceware.org/binutils/docs-2.20/ld/Output-Section-LMA.html
-  CurOutSec->setLMAOffset(LMAOffset);
+  Expr LMAExpr = CurLMA.first;
+  if (LMAExpr)
+    CurOutSec->setLMAOffset(LMAExpr(CurLMA.second) - CurLMA.second);
 }
 
 template <class ELFT> void LinkerScript<ELFT>::process(BaseCommand &Base) {
   // This handles the assignments to symbol or to a location counter (.)
   if (auto *AssignCmd = dyn_cast<SymbolAssignment>(&Base)) {
     assignSymbol(AssignCmd, true);
     return;
   }
MemoryRegion *LinkerScript<ELFT>::findMemoryRegion(OutputSectionCommand *Cmd,
   return nullptr;
 }
 
 // This function assigns offsets to input sections and an output section
 // for a single sections command (e.g. ".text { *(.text); }").
 template <class ELFT>
 void LinkerScript<ELFT>::assignOffsets(OutputSectionCommand *Cmd) {
   if (Cmd->LMAExpr)
-    LMAOffset = Cmd->LMAExpr(Dot) - Dot;
+    CurLMA = {Cmd->LMAExpr, Dot};
   OutputSectionBase *Sec = findSection<ELFT>(Cmd->Name, *OutputSections);
   if (!Sec)
     return;
 
   if (Cmd->AddrExpr && Sec->Flags & SHF_ALLOC)
     setDot(Cmd->AddrExpr);
 
   // Handle align (e.g. ".foo : ALIGN(16) { ... }").

test/ELF/linkerscript/at-addr.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { . = 0x1000; \
+# RUN:  .aaa : AT(ADDR(.aaa) - 0x500) { *(.aaa) } \
+# RUN:  .bbb : AT(ADDR(.bbb) - 0x500) { *(.bbb) } \
+# RUN:  .ccc : AT(ADDR(.ccc) - 0x500) { *(.ccc) } \
+# RUN: }" > %t.script
+# RUN: ld.lld %t --script %t.script -o %t2
+# RUN: llvm-readobj -program-headers %t2 | FileCheck %s
+
+# CHECK:      ProgramHeaders [
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_PHDR
+# CHECK-NEXT:     Offset: 0x40
+# CHECK-NEXT:     VirtualAddress: 0x40
+# CHECK-NEXT:     PhysicalAddress: 0x40
+# CHECK-NEXT:     FileSize:
+# CHECK-NEXT:     MemSize:
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment: 8
+# CHECK-NEXT:   }
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x0
+# CHECK-NEXT:     VirtualAddress: 0x0
+# CHECK-NEXT:     PhysicalAddress: 0x0
+# CHECK-NEXT:     FileSize:
+# CHECK-NEXT:     MemSize:
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:       PF_X
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment:
+# CHECK-NEXT:   }
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x1000
+# CHECK-NEXT:     VirtualAddress: 0x1000
+# CHECK-NEXT:     PhysicalAddress: 0xB00
+# CHECK-NEXT:     FileSize: 8
+# CHECK-NEXT:     MemSize: 8
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:       PF_X
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment:
+# CHECK-NEXT:   }
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x1008
+# CHECK-NEXT:     VirtualAddress: 0x1008
+# CHECK-NEXT:     PhysicalAddress: 0xB08
+# CHECK-NEXT:     FileSize: 8
+# CHECK-NEXT:     MemSize: 8
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:       PF_X
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment: 4096
+# CHECK-NEXT:   }
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_LOAD
+# CHECK-NEXT:     Offset: 0x1010
+# CHECK-NEXT:     VirtualAddress: 0x1010
+# CHECK-NEXT:     PhysicalAddress: 0xB10
+# CHECK-NEXT:     FileSize: 9
+# CHECK-NEXT:     MemSize: 9
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:       PF_X
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment: 4096
+# CHECK-NEXT:   }
+# CHECK-NEXT:   ProgramHeader {
+# CHECK-NEXT:     Type: PT_GNU_STACK
+# CHECK-NEXT:     Offset:
+# CHECK-NEXT:     VirtualAddress: 0x0
+# CHECK-NEXT:     PhysicalAddress: 0x0
+# CHECK-NEXT:     FileSize:
+# CHECK-NEXT:     MemSize:
+# CHECK-NEXT:     Flags [
+# CHECK-NEXT:       PF_R
+# CHECK-NEXT:       PF_W
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Alignment: 0
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+.global _start
+_start:
+ nop
+
+.section .aaa, "a"
+.quad 0
+
+.section .bbb, "a"
+.quad 0
+
+.section .ccc, "a"
+.quad 0