[llvm-dev] Is it time to allow StringRef to be constructed from nullptr?
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Sun Sep 25 16:46:46 PDT 2016
> On Sep 25, 2016, at 4:11 PM, Zachary Turner <zturner at google.com> wrote:
>
> I thought about doing something like that, but most compilers will fold a call to strlen on a string literal into a constant anyway, so in practice I don't think it matters much. I know Clang does, and I tested MSVC and it does too.
To be clear: I’m not worried that this would add cost to the literal case. And the other change I’m suggesting is (relatively) orthogonal and isn’t a blocker for what you want to do.
—
Mehdi
>
> D:\>type strlen.cpp
> #include <string.h>
> #include <stdio.h>
>
> int main(int argc, char **argv) {
> int x = strlen("This is a test");
> printf("%d", x);
> return 0;
> }
>
> D:\>cl /O2 strlen.cpp
> Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24213.1 for x86
> Copyright (C) Microsoft Corporation. All rights reserved.
>
> strlen.cpp
> Microsoft (R) Incremental Linker Version 14.00.24213.1
> Copyright (C) Microsoft Corporation. All rights reserved.
>
> /out:strlen.exe
> strlen.obj
>
> D:\>dumpbin strlen.obj /disasm | grep -C 5 main
> 00000018: FF 30 push dword ptr [eax]
> 0000001A: E8 00 00 00 00 call ___stdio_common_vfprintf
> 0000001F: 83 C4 18 add esp,18h
> 00000022: C3 ret
>
> _main:
> 00000000: 6A 0E push 0Eh
> 00000002: 68 00 00 00 00 push offset ??_C at _02DPKJAMEF@?$CFd?$AA@
> 00000007: E8 00 00 00 00 call _printf
> 0000000C: 83 C4 08 add esp,8
> 0000000F: 33 C0 xor eax,eax
>
>
> Also, IANALL, but I don't believe you can overload on const char* vs. const char (&T)[N]. If you have both overloads, a string literal and char array will still select the const char* overload, at least in the tests I attempted.
>
> On Sun, Sep 25, 2016 at 3:58 PM Pete Cooper <peter_cooper at apple.com <mailto:peter_cooper at apple.com>> wrote:
>> On Sep 25, 2016, at 1:49 PM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>>
>>> On Sep 25, 2016, at 9:10 AM, Zachary Turner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>
>>> While porting LLDB over to StringRef, I am continuously running into difficulties caused by the fact that StringRef cannot be constructed from nullptr. So I wanted to see peoples' thoughts on removing this restriction from StringRef. To be clear, I'm only using LLDB as a motivating example, but I'm not requesting that it be done because LLDB is some kind of special case. If it is to be done it should be on its own merits. That said, here is some context:
>>>
>>> LLDB has a lot of functions that look like this:
>>>
>>> void foo(const char *, Bar, const char *).
>>>
>>> I'm trying to port these to functions that look like this:
>>>
>>> void foo(StringRef, Bar, StringRef).
>>>
>>> Often times the parameters are string literals or char arrays, but equally often they are another const char* that got passed into the calling function, or a return value from a CRT function like strstr(), or many other possible sources. This latter category presents a problem for porting code to StringRef, because if I simply change the function signature and fix up compile errors, I will probably have introduced a bug because hundreds of callers will now be implicitly converting from const char* to StringRef, leaving open the possibility that one of those was null.
>>>
>>> To work around this, I've started doing the following every time I port a function:
>>>
>>> void foo(const char *, Bar, const char*) = delete;
>>>
>>> This is pretty hackish, but it gets the job done. At least the compiler warns me and forces me to go inspect every callsite where there's an implicit conversion. Unfortunately it also makes for extremely verbose code. Now instead of:
>>>
>>> foo("bar", baz, "buzz")
>>>
>>> I have to write
>>>
>>> foo(StringRef("bar"), baz, StringRef("buzz"))
>>>
>>> even for string literals and char arrays, which will obviously never be null! If StringRef would handle a null argument gracefully, it would make my life much easier.
>>>
>>>
>>> With that out of the way, here are some reasons I can see to allow StringRef accept null to its constructor which are independent of LLDB and stand on their own merit.
>>>
>>> 1) std::string_view<> can be constructed with null. I don't know when we will be able to use std::string_view<>, but there's a chance that at some point in the future we may wish to remove StringRef in favor of string_view. That day isn't soon, but in any case, it will be easier if our assumptions are the same.
>>>
>>> 2) [nullptr, nullptr+0) is a valid range. Why shouldn't we be able to construct a StringRef from an otherwise perfectly valid range?
>>>
>>> 3) StringRef() can already be constructed from nullptr (!) Surprised? That's what happens when you invoke the default constructor. It happily initializes the internal Data with null. So why not allow the same behavior when invoking the const char * constructor?
>>>
>>>
>>> Thoughts?
>>
>>
>> As a tangent: I don’t like the fact that StringRef is implicitly built out of “const char *”, this is calling strlen() and because it is implicit folks don’t realize when they go from string -> char * -> StringRef.
>> I rather have this constructor explicit, and provide an implicit one for string literal.
>
> I wonder if we could change that call site to be deleted (or at least explicit), and add support for literal strings with a StringRef version of this:
>
> /// Construct an ArrayRef from a C array.
> template <size_t N>
> /*implicit*/ LLVM_CONSTEXPR ArrayRef(const T (&Arr)[N])
> : Data(Arr), Length(N) {}
>
> This way we’ll avoid the strlen on quoted strings which is the common case anyway, and then can see how many other cases we have from const char* remaining.
>
> Pete
>
>>
>> To come back to your point, I’m not sure if we should leave the internal pointer null or always set it to “”? This would provide the guarantee that dereferencing a StringRef is always valid without checking.
>>
>> —
>> Mehdi
>>
>
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