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

[Shankar Easwaran via Phabricator via llvm-commits]

shankare 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:
> > > > 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.
> How would objdump or anytool be able to find out that an executable section is having data ? Without adding a mapping symbol I believe it would be hard for objdump to figure out that information. 
Sure, We can decide if we would want to mimic that behavior probably later then. 


https://reviews.llvm.org/D30724