[PATCH] D126324: [clang] Allow const variables with weak attribute to be overridden

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D126324#3537373 <https://reviews.llvm.org/D126324#3537373>, @wanders wrote:

> In D126324#3536785 <https://reviews.llvm.org/D126324#3536785>, @aaron.ballman wrote:
>
>> The changes so far look sensible, but I think we should add some more tests for a few situations. 1) Using a const weak symbol as a valid initializer should be diagnosed (with a warning? with an error?) so users are alerted to the behavioral quirks. 2) Using a const weak symbol in a constant expression context should probably be an error, right? e.g.,
>
> I do think we have already diagnose/fail on all the relevant cases here. And have tests for them, like
>
>   extern const int weak_int __attribute__((weak));
>   const int weak_int = 42;
>   int weak_int_test = weak_int; // expected-error {{not a compile-time constant}}
>
> in clang/test/Sema/const-eval.c
>
> But I'll have another look to make sure we have proper coverage.

Thanks for double-checking it -- if we've got reasonable coverage, then that's great (nothing else needs to happen).

>> Also, this definitely could use a release note so users know about the behavioral change.
>
> Happy to write one, but not sure what it should say.
> The "only" change in behavior is that frontend no longer tells backend it is allowed to optimize based on initializer value. Frontend behavior is intended to be kept exactly the same (it already is restrictive enough).

I thought this was more disruptive than it actually is, so I feel less strongly. But if you wanted to add one, I'd put it under the `Attribute Changes in Clang` heading and something along the lines of what you just wrote about the backend no longer being allowed to optimize in some circumstances.

>> Do you have any ideas on how much code this change is expected to break? (Is it sufficient enough that we want to have an explicit deprecation period of the existing behavior before we switch to the new behavior?)
>
> I don't really know much code out there would be affected. As mentioned in the discourse thread https://discourse.llvm.org/t/weak-attribute-semantics-on-const-variables/62311/7 I did some grepping in open source projects I could find. This was biased towards looking at C code.
> I could see two different uses of const+weak:
>
> 1. For defining a constant in a header file (which makes that symbol end up in many object files, making it weak avoids multiple definition link errors)
> 2. To define some default configuration/identificationstring which is overridden by a strong symbol
>
> There might of course be other uses of weak that I'm not aware of or could find.
>
> But these two cases I want to think are fine with the proposed change:
>
> For 1. we wouldn't alter behavior of code just speed due to optimization being prevented. (I think these uses want to use "selectany" attribute instead)
>
> For 2. this change should actually fix a bug where functions in the same translation unit that provided the default was using the default rather than overridden value.
>
> Here are some examples I found:
> In u-boot they do this https://source.denx.de/u-boot/u-boot/-/blob/master/common/hwconfig.c#L71 and I think (havn't verified) clang/llvm would optimize the `strlen(cpu_hwconfig)` on line 103 to `0`. So here I believe this code currently has incorrect behavior when compiled with clang.
>
> Arduino for esp does this constant in header https://github.com/esp8266/Arduino/blob/master/variants/generic/common.h#L79 so that would cause an extra load of the value from memory for everyone blinking the LED using the old deprecaded name for that pin.
>
> Here is a c++ example https://github.com/ClickHouse/ClickHouse/blob/master/src/Common/Allocator.cpp#L11 which I think it might not allow overriding the variable with a strong definition as intended when compiling with clang.
>
> The case that motivated me to look into this is like:
>
>   const char VERSION[] __attribute__ ((weak))= "devel";
>   
>   int is_devel_version(void) {
>       return !strcmp (VERSION, "devel");
>   }
>
> and when "VERSION" gets "weak_odr" linkage "is_devel_version" gets optimized to "return 1".  Even if VERSION gets replaced with a strong value later in link phase.

Excellent, thank you!

I am comfortable moving forward with this.


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