[PATCH] D44780: [ELF] - Implement linker script OVERLAYs.

[George Rimar via Phabricator via llvm-commits]

grimar added inline comments.


================
Comment at: ELF/LinkerScript.h:315
+  // that were seen in overlay descriptions.
+  llvm::DenseSet<OutputSection *> OverlaySections;
 };
----------------
espindola wrote:
> This could be a bit in OutputSection, no?
Yes. Ok.


================
Comment at: ELF/ScriptParser.cpp:451
+    if (Prev)
+      Cmd->LMAExpr = [=] { return Prev->LMAExpr().getValue() + Prev->Size; };
+    else
----------------
espindola wrote:
> Are we missing the alignment?
GNU ld does not seem to respect the alignment here.

For example for code and script:
```
.section .aaa, "a"; 
.byte 0; 

.section .bbb, "a"; 
.align 4; 
.byte 0; 
```

```
SECTIONS {
 . = 0x1000;
  OVERLAY 0x1000 : AT ( 0x4000 ) {
    .aaa { *(.aaa) } 
    .bbb { *(.bbb) } 
  } 
}
```

Output is:


```
Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .aaa              PROGBITS         0000000000001000  00001000
       0000000000000001  0000000000000000   A       0     0     1
  [ 2] .bbb              PROGBITS         0000000000001000  00002000
       0000000000000001  0000000000000000   A       0     0     4

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000004000
                 0x0000000000000001 0x0000000000000001  R      1000
  LOAD           0x0000000000002000 0x0000000000001000 0x0000000000004001
                 0x0000000000000001 0x0000000000000001  R      1000

 Section to Segment mapping:
  Segment Sections...
   00     .aaa 
   01     .bbb 

```

And I might be missing something, but it seems correct to me. I see no reason to
waste space with alignments for OVERLAYs, because overlay manager is responsible to
copy sections in and out of the runtime memory address and therefore I think alignment
is just unuseful here.
(4.6.9 Overlay Description, https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html)


https://reviews.llvm.org/D44780