section type conflict with clang, not with gcc

[llvmbot]

Bugzilla Link	48696
Version	trunk
OS	Linux
Attachments	code to reproduce.
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid,@whisperity

Extended Description

We ran into an issue when adding a section attribute for certain symbols:

[[gnu::section(".bss")]] const int Stuff::xi = rand();
[[gnu::section(".bss")]] std::tr1::unordered_map<int,int> uom;

Full example code attached.

error I get when compiling:

$ clang++ -O2 -pipe -g -fno-omit-frame-pointer -std=c++11 main.cc stuff.cc
stuff.cc:8:59: error: 'uom' causes a section type conflict with 'xi'
[[gnu::section(".bss")]] std::tr1::unordered_map<int,int> uom;
                                                          ^
stuff.cc:6:43: note: declared here
[[gnu::section(".bss")]] const int Stuff::xi = rand();
                                          ^
1 error generated.

Compiled without the section attributes result is that they are both put in .bss, but when explicitly defining them, clang++ throws a section type conflict.

Tried both with llvm 8.0 and 12.0. Also tried with -fno-zero-initialized-in-bss, in that case they were put in .data, but I got the same error when I added the attribute.

The same code compiles and runs with g++ (tried with versions 5.4.0 and 7.5.0).

[zygoloid]

Confirmed. From SemaDecl.cpp:

    if (var->getType().isConstQualified())
      Stack = &ConstSegStack;

... which is nonsense. Similarly, this trivial testcase is miscompiled:

int f() { return 1; }
[[gnu::section(".foo")]] const int n = f(); 
int main() { return n; }

... because the .foo section is marked read-only, and then modified during program startup.

GCC appears to check whether the variable is of a constant type (either const-qualified with no mutable fields, or a reference) and has constant initialization and trivial destruction (it should probably check for constant destruction, but apparently doesn't!), when deciding whether the section is implicitly read-only. We should do the same.

[AaronBallman]

I believe these issues were resolved in Clang 13: https://godbolt.org/z/eqqaPa38h and https://godbolt.org/z/ThzMaYdM1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

Details

| | | | --- | --- | | Bugzilla Link | [48696](https://llvm.org/bz48696) | | Version | trunk | | OS | Linux | | Attachments | [code to reproduce.](https://user-images.githubusercontent.com/60944935/143761623-0b475861-b737-483e-9e42-c774e4d548c6.gz) | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@zygoloid,@whisperity |

Extended Description

We ran into an issue when adding a section attribute for certain symbols:

[[gnu::section(".bss")]] const int Stuff::xi = rand();
[[gnu::section(".bss")]] std::tr1::unordered_map<int,int> uom;

Full example code attached.

error I get when compiling:

$ clang++ -O2 -pipe -g -fno-omit-frame-pointer -std=c++11 main.cc stuff.cc
stuff.cc:8:59: error: 'uom' causes a section type conflict with 'xi'
[[gnu::section(".bss")]] std::tr1::unordered_map<int,int> uom;
                                                          ^
stuff.cc:6:43: note: declared here
[[gnu::section(".bss")]] const int Stuff::xi = rand();
                                          ^
1 error generated.

Compiled without the section attributes result is that they are both put in .bss, but when explicitly defining them, clang++ throws a section type conflict.

Tried both with llvm 8.0 and 12.0. Also tried with -fno-zero-initialized-in-bss, in that case they were put in .data, but I got the same error when I added the attribute.

The same code compiles and runs with g++ (tried with versions 5.4.0 and 7.5.0).