https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94485
Bug ID: 94485
Summary: [10.0.1, c++2a] g++ optimizes away the code, accepts
arbitrary inline asm
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: dimitri.gorokhovik at free dot fr
Target Milestone: ---
Created attachment 48194
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48194&action=edit
Sample code illustrating the sighting.
Version: gcc (GCC) 10.0.1 20200404 (experimental)
command line:
~/gcc-trunk/dist/bin/g++ -std=c++2a -O3 -Wall -Wextra -W bug-202004041709.cpp
-o bug-1 && ./bug-1
The attached file (apologies, still big) contains the following code (line
425):
constexpr iterator (tesselation const& me)
: cube_ { me.cube_ },
inner_ { me.as_parent ().begin () },
tess_ {},
outer_ { tess_ }
{
if (inner_)
{
tess_ = tesselation_of_two_cubes { cube_, *inner_ };
asm("before:");
#if 0 // breaks only with -O3
auto const tmp { tess_.begin () };
asm("middle:");
outer_ = tmp;
^^^^^^^^^^^^
#else // breaks with -O3, -O1, fixes with -fsanitize=undefined|null
outer_ = { tess_.begin () };
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
asm("after:");
#endif
inner_.next ();
if (! outer_) throw 749;
};
};
(Minor remark: 'asm' statements in this constexpr method raise no brows. I
tried putting there insns that require evaluation, seems anything is silently
accepted. The same asm statements are rejected in a more simple constexpr
code.)
It seems, some optimization levels completely remove the highlighted assignment
so that 'throw 749' is executed (main() prints it then). Other passes keep the
copying code and produce expected result. This over-optimization can be fixed
by enabling certain sanitizers: undefined, null, vptr.
The asm code produced:
-- broken (compiled with -O3 or -O1):
#APP
# 425 "bug-1.cpp" 1
before:
# 0 "" 2
#NO_APP
movq %r12, 216(%rsp)
#APP
# 433 "bug-1.cpp" 1
after:
# 0 "" 2
#NO_APP
-- working (compiled with -O0 or -O1 or with -fsanitizer=null):
#APP
# 425 "bug-1.cpp" 1
before:
# 0 "" 2
#NO_APP
xorl %eax, %eax
cmpb $0, 43(%rbx)
movq %r14, 64(%rsp)
movw %ax, 72(%rsp)
movb $0, 74(%rsp)
je .L73
cmpb $0, 63(%rbx)
je .L73
cmpb $0, 83(%rbx)
sete %al
.L68:
movb %al, 75(%rsp)
movq 64(%rsp), %rax
movdqa 64(%rsp), %xmm6
movq %rax, 88(%rbx)
movl 72(%rsp), %eax
movaps %xmm6, 96(%rsp)
movl %eax, 96(%rbx)
#APP
# 433 "bug-1.cpp" 1
after:
# 0 "" 2
#NO_APP
