[llvm] r239254 - Remove global std::string. NFC.

[Yaron Keren]

With a helper function we can get a global StringRef without a constructor
call, try this:

class StringRef {
...
    template <unsigned N>
    static constexpr StringRef create(char const(&Str)[N]) {
      return StringRef(Str, N - 1);
    }
...
}

...
const char hello[] = "hello";
const llvm::StringRef Hello = llvm::StringRef::create(hello);
bool ggg(llvm::StringRef s) { return s != Hello; }



2015-06-08 0:11 GMT+03:00 Yaron Keren <yaron.keren at gmail.com>:

> Yes, I see. I had tried a small example with clang r239218, below. The
> global StringRef indeed requires a global constructor which is bad. The
> strlen call in the const char * version is optimized away even in -O2. It
> would be very nice if StringRef constructor would be optimized the same.
>
> The C++ code
>
> #include "llvm/ADT/StringRef.h"
> const char *const hello = "hello";
> bool fff(llvm::StringRef s) { return s != hello; }
> llvm::StringRef Hello(hello);
> bool ggg(llvm::StringRef s) { return s != Hello; }
>
> compiled with
>
> clang -O3 a.cpp -std=c++11 -I c:\llvm\include -S -emit-llvm
>
>
> 2015-06-07 23:31 GMT+03:00 Benjamin Kramer <benny.kra at gmail.com>:
>
>>
>> > On 07.06.2015, at 22:26, Yaron Keren <yaron.keren at gmail.com> wrote:
>> >
>> > > Which doesn't matter at all because a StringRef is just a pointer +
>> size and the size can be determined statically
>> > >for GCSafepointPollName. The StringRef construction is folded away
>> completely at clang -O3.
>> >
>> > A global variable StringRef(const char *) does not get optimized the
>> same way?
>>
>> Exactly. This is a deficiency in LLVM's optimizer but one that's very
>> hard to fix. Clang emits a writeable global and a function to initialize it
>> which is called by the linker at startup. The optimizer tries to statically
>> evaluate the function but that often fails.
>>
>> - Ben
>>
>> > 2015-06-07 22:44 GMT+03:00 Benjamin Kramer <benny.kra at gmail.com>:
>> >
>> > > On 07.06.2015, at 21:33, Yaron Keren <yaron.keren at gmail.com> wrote:
>> > >
>> > > If StringRef (const char *) is optimized away than StringRef
>> GCSafepointPollName is better.
>> >
>> > This creates the global in .bss and writes to it in a global
>> constructor at clang -O3. This also means that LLVM cannot constant fold
>> accesses to the global variable.
>> >
>> > > If StringRef (const char *) is not optimized away then
>> > >
>> > > static bool isGCSafepointPoll(Function &F) {
>> > >   return F.getName().equals(GCSafepointPollName);
>> > > }
>> > >
>> > > will construct a StringRef on every call to isGCSafepointPoll instead
>> of once globally.
>> >
>> > Which doesn't matter at all because a StringRef is just a pointer +
>> size and the size can be determined statically for GCSafepointPollName. The
>> StringRef construction is folded away completely at clang -O3.
>> >
>> > - Ben
>> >
>> > >
>> > >
>> > > 2015-06-07 22:13 GMT+03:00 Benjamin Kramer <benny.kra at gmail.com>:
>> > >
>> > > > On 07.06.2015, at 20:24, Yaron Keren <yaron.keren at gmail.com> wrote:
>> > > >
>> > > > I don't think any of this would actually matter since all would be
>> optimized away, but if anything, it's probably best to match
>> GCSafepointPollName two code users which expect a StringRef by making this
>> a static StringRef.
>> > > >
>> > > > In general, unless a null-terminated C string is actually required
>> (for example, for OS API) StringRefs are always superior  to const char *.
>> Just think about the easy mistake S == "some string".
>> > >
>> > > I don't know of a compiler that can optimize away global
>> std::strings, it is very hard and probably not feasible without a
>> domain-specific C++ optimizer. Clang for example emits a global constructor
>> and destructor for this example which is badness.
>> > >
>> > > I agree that StringRef is superior but not in a global variable.
>> LLVM's globalopt can optimize them away if you get lucky, but I don't want
>> to rely on that. The only acceptable way for complex types in global
>> constants is constexpr, but making StringRef's ctor constexpr isn't
>> possible because of the strlen call in there.
>> > >
>> > > We also have warnings for common char* misuse in clang.
>> > >
>> > > - Ben
>> > >
>> > > >
>> > > >
>> > > > 2015-06-07 19:36 GMT+03:00 Benjamin Kramer <
>> benny.kra at googlemail.com>:
>> > > > Author: d0k
>> > > > Date: Sun Jun  7 11:36:28 2015
>> > > > New Revision: 239254
>> > > >
>> > > > URL: http://llvm.org/viewvc/llvm-project?rev=239254&view=rev
>> > > > Log:
>> > > > Remove global std::string. NFC.
>> > > >
>> > > > Modified:
>> > > >     llvm/trunk/lib/Transforms/Scalar/PlaceSafepoints.cpp
>> > > >
>> > > > Modified: llvm/trunk/lib/Transforms/Scalar/PlaceSafepoints.cpp
>> > > > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/PlaceSafepoints.cpp?rev=239254&r1=239253&r2=239254&view=diff
>> > > >
>> ==============================================================================
>> > > > --- llvm/trunk/lib/Transforms/Scalar/PlaceSafepoints.cpp (original)
>> > > > +++ llvm/trunk/lib/Transforms/Scalar/PlaceSafepoints.cpp Sun Jun  7
>> 11:36:28 2015
>> > > > @@ -496,7 +496,7 @@ template <typename T> static void unique
>> > > >    }
>> > > >  }
>> > > >
>> > > > -static std::string GCSafepointPollName("gc.safepoint_poll");
>> > > > +static const char *const GCSafepointPollName = "gc.safepoint_poll";
>> > > >
>> > > >  static bool isGCSafepointPoll(Function &F) {
>> > > >    return F.getName().equals(GCSafepointPollName);
>> > > >
>> > > >
>> > > > _______________________________________________
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>> > > > llvm-commits at cs.uiuc.edu
>> > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>> > > >
>> > >
>> > >
>> >
>> >
>>
>>
>
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