[Bug c++/21222] A function with a particular number of arguments corrupts a value calculated on the stack when stepping through the function

["kungfujesus06 at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21222

--- Comment #8 from Adam Stylinski <kungfujesus06 at gmail dot com> ---
(In reply to Keith Seitz from comment #7)
> As you note, this is likely a display problem.
> 
> So the next question is, what does "info addr lambda" say? I wonder if the
> debug info is incorrect.
> 
> On my system (which does not support VEX):
> 
> 12	    float lambda = C_LIGHT / f;
> (gdb) n
> 13	    printf("lambda = %f\n", lambda);
> (gdb) p lambda
> $1 = 0.031557098
> (gdb) info addr lambda
> Symbol "lambda" is a complex DWARF expression:
>      0: DW_OP_fbreg -20
> .
> (gdb) inf frame
> Stack level 0, frame at 0x7fffffffd990:
>  rip = 0x400679 in testFunc (21222.cc:13); saved rip = 0x4006dd
>  called by frame at 0x7fffffffd9d0
>  source language c++.
>  Arglist at 0x7fffffffd980, args: a=0x0, n=0, sx=0x0, sy=0x0, rc=0x0, nr=0, 
>     b=4, f=9.50000026e+09, c=0x0, d=0x0, e=0x0, gx=0x0, gy=0x0
>  Locals at 0x7fffffffd980, Previous frame's sp is 0x7fffffffd990
>  Saved registers:
>   rbp at 0x7fffffffd980, rip at 0x7fffffffd988
> (gdb) p *(float*)(0x7fffffffd990-20)
> $2 = 0.031557098
> 
> What compiler/version are you using?

I'm using g++ 6.3.1.  I believe this is more than a display issue, as if I
attach the debugger here when the binary performs the printf I see the
corrupted value.  There's definitely something going on with GDB & VEX
instructions (this doesn't happen if using the legacy SSE ones).  


Breakpoint 1, testFunc (a=0x0, n=0, sx=0x0, sy=0x0, rc=0x0, nr=0, b=4,
f=9.50000026e+09, c=0x0, d=0x0, e=0x0, gx=0x0, gy=0x0) at testfunc.cpp:12
12          float lambda = C_LIGHT / f;
(gdb) n
13          printf("lambda = %f\n", lambda);
(gdb) info addr lambda
Symbol "lambda" is a complex DWARF expression:
     0: DW_OP_fbreg -20
.

(gdb) inf frame
Stack level 0, frame at 0x7fffffffde80:
 rip = 0x400562 in testFunc (testfunc.cpp:13); saved rip = 0x4005c6
 called by frame at 0x7fffffffdec0
 source language c++.
 Arglist at 0x7fffffffde70, args: a=0x0, n=0, sx=0x0, sy=0x0, rc=0x0, nr=0,
b=4, f=9.50000026e+09, c=0x0, d=0x0, e=0x0, gx=0x0, gy=0x0
 Locals at 0x7fffffffde70, Previous frame's sp is 0x7fffffffde80
 Saved registers:
  rbp at 0x7fffffffde70, rip at 0x7fffffffde78

(gdb) p *(float*)(0x7fffffffde80 - 20)
$1 = -5.38359187e+25

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