https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95663
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |law at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
Component|c++ |tree-optimization
Keywords| |missed-optimization
Target| |x86_64-*-* i?86-*-*
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I suppose the C++ standard says static_cast<Derived *>(nullptr) == nullptr and
we literally follow that. Note it will make a difference for very large
objects (and thus very large offsets added) which may end up acccessing
actually
mapped memory so IMHO what clang does by default is a security risk.
Now, what we should eventually improve is the code generated in the isolated
path. On GIMPLE we retain the load:
<bb 4> [count: 0]:
_7 ={v} MEM[(struct Derived *)0B].D.2340.y;
__builtin_trap ();
(because it can trap). The use of the cold section for the
ud2 is probably also bad since it will cause a larger jump instruction
where very likely
testq %rdi, %rdi
jne .L2
ud2
.L2:
movl (%rdi), ...
would be both faster and smaller. For reference the generated code:
_Z5fieldP5Base2:
.LFB1:
.cfi_startproc
testq %rdi, %rdi
je .L2
movl (%rdi), %eax
ret
.cfi_endproc
.section .text.unlikely
.cfi_startproc
.type _Z5fieldP5Base2.cold, @function
_Z5fieldP5Base2.cold:
.LFSB1:
.L2:
movl 4, %eax
ud2
CCing Jeff for the RTL side representation - IIRC we have some special
CFG magic for gcc_unreachable, not sure if what we end up with
trap() matches that or if we should adjust this somehow. Currently DCE
marks the load as always necessary because it seems isolate-paths makes the
load
volatile:
/* We want the NULL pointer dereference to actually occur so that
code that wishes to catch the signal can do so.
...
fair enough - but as you see above we dereference not NULL but some
derived constant which might not actually trap. I wonder if it is
more useful/safe to replace the load with a plain *(char *)0?
Note I don't think what clang does here is reasonable.
