[Bug gdb/14261] New: Break point not set at the 1st executableinstruction in case of code compiled with clang on ARM Architecture.

["karthikthecool at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14261

             Bug #: 14261
           Summary: Break point not set at the 1st executable instruction
                    in case of code compiled with clang on ARM
                    Architecture.
           Product: gdb
           Version: 7.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
        AssignedTo: unassigned@sourceware.org
        ReportedBy: karthikthecool@gmail.com
    Classification: Unclassified


Created attachment 6456
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6456
Assembly files with R(0123) which works fine and Assembly generated by clang
which uses R(4-12) as well.

Dear Developers,
We are using clang(LLVM) compiled binary on ARM and debugging the same using
GDB. When a break point is set at a function, it is set at the 1st line instead
of 1st executable instruction.
E.g.
For e.g. In the below function-

int main()          //line 1
{                   //line 2
                    //line 3
  int j =0;         //line 4
  return j;         //line 5
}                   //line 6


When we compile the above code and run gdb with the following commands-

gdb a.out
break main

The break point is set at line 2 instead of line 4.


Upon Debugging we found that in GDB Arm-tdep.c -

static CORE_ADDR
arm_skip_prologue (struct gdbarch *gdbarch, CORE_ADDR pc)


      if ((inst & 0xffffc000) == 0xe5cd0000    /* strb r(0123),[sp,#nn] */
      || (inst & 0xffffc0f0) == 0xe1cd00b0    /* strh r(0123),[sp,#nn] */
      || (inst & 0xffffc000) == 0xe58d0000)    /* str  r(0123),[sp,#nn] */
    continue;

We skip the prologue only in case strb uses registers r0,r1,r2 or r3 in
prologue code. 

clang (LLVM) generates strb instruction with r4,r5...etc not just r(0123) in
prologue code.

In clang case the usage of any other register other than r(0123) in
strb/strh/str instruction is resulting in generation of wrong prologue_end
resulting in break point not being set at 1st executable instruction.When we
modified the strb/strh/str instructions to use r(0123) gdb functionality of
break/list/watch functions correctly.

Attached is the assembly and source code of a function compiled using clang in
which r12/ r4 is used in strb/strh/str/ldr etc instructions used during
parameter assignment 
which results in failure of the above check in arm_skip_prologue resulting in
wrong prologue end info.



We would like to know if we are checking against r(0123) for some specific
purpose or because some specification mandates this usage ?

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