[PATCH] D30724: Dont emit Mapping symbols for sections that contain only data.

[Tim Northover via Phabricator via llvm-commits]

t.p.northover added inline comments.


================
Comment at: lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:630-631
+    // executable permissions.
+    if (LastEMS != EMS_None ||
+        (cast<MCSectionELF>(getCurrentSectionOnly()))->UseCodeAlign()) {
+      LastEMSInfo->resetInfo();
----------------
shankare wrote:
> t.p.northover wrote:
> > shankare wrote:
> > > t.p.northover wrote:
> > > > I still don't see why we need this `UseCodeAlign` special case, I'm afraid.
> > > The only time that we want to emit a Data symbol is if the section was set to be executable but it contains data. 
> > > 
> > > 
> > Why? The ABI says nothing about an executable bit:
> > 
> >    "A section must have a mapping symbol defined at the beginning of the section; however, if the section contains only data then the mapping symbol may be omitted."
> If the section is marked as containing instructions(on elf using the section flag, UseCodeAlign checks for section permission) and it contains data, dont we need a $d to specifiy beginning of data. 
> 
> GCC does it as the below test shows :-
> 
> The section foobar has a section flag below to specify that it contains instructions.
> 
> $ cat b.s
>         .section        .foobar,"ax",%progbits
>          .word 32
> 
> $ readelf -s b.o
> 
> Symbol table '.symtab' contains 7 entries:
>    Num:    Value  Size Type    Bind   Vis      Ndx Name
>      0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
>      1: 00000000     0 SECTION LOCAL  DEFAULT    1
>      2: 00000000     0 SECTION LOCAL  DEFAULT    2
>      3: 00000000     0 SECTION LOCAL  DEFAULT    3
>      4: 00000000     0 SECTION LOCAL  DEFAULT    4
>      5: 00000000     0 NOTYPE  LOCAL  DEFAULT    4 $d
>      6: 00000000     0 SECTION LOCAL  DEFAULT    5
> 
Not according to how I read the ABI. It's quite possible GNU as adds unneeded symbols in that case, but  I don't think "because GCC does it" is reason enough for us to add this complexity when we've got an actual document telling us what we should be doing.


https://reviews.llvm.org/D30724