[libc] Define long long limits if not defined

[abrachet]

Summary

Some older gcc toolchains don't define these on 32 bit platforms. This
is a problem for pigweed which uses an older gcc toolchain and targets
32 bit.

[michaelrj]

this patch looks similar to https://reviews.llvm.org/D156961

[abrachet]

Unfortunately that won't work for gcc whose limits.h https://gnu.googlesource.com/gcc/+/refs/heads/trunk/gcc/glimits.h doesn't use include_next. The compiler header will be chosen first.

[michaelrj]

alright, in that case since this is exclusively for internal use then I'm okay with having our own definitions for these constants. I'd prefer to use constexpr variables for this instead of macros, since those provide better type checking.

src/__support/CPP/limits.h

16 ↗

(On Diff #547236)

While in almost every case this is safe, I'd still prefer not to assume that long long is 64 bits. Either (sizeof(long long) * 8) - 1 or some other way to calculate the size would be better.

[lntue]

libc/src/__support/CPP/limits.h

19

This constant name in the header is a bit too generic. Maybe you can name it:

constexpr size_t LLONG_BIT_WIDTH = sizeof(long long) * 8;

and use LLONG_BIT_WIDTH - 1 instead of size_bits below.

Or we can add the templates BitWidth<T>::value = sizeof(T) * 8 and MostSignificantBitExponent<T>::value = BitWidth<T>::value - 1 or something like this, as they are actually used in many places.

libc/src/__support/CPP/limits.h

 //===-- A self contained equivalent of std::limits --------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_SUPPORT_CPP_LIMITS_H
 #define LLVM_LIBC_SRC_SUPPORT_CPP_LIMITS_H
 
 #include <limits.h>
 
 namespace __llvm_libc {
 namespace cpp {
 
+// Some older gcc distributions don't define these for 32 bit targets.
+#ifndef LLONG_MAX
+constexpr size_t LLONG_BIT_WIDTH = sizeof(long long) * 8;
+constexpr long long LLONG_MAX = ~0LL ^ (1LL << LLONG_BIT_WIDTH - 1);
+constexpr long long LLONG_MIN = 1LL << LLONG_BIT_WIDTH - 1;
+constexpr unsigned long long ULLONG_MAX = ~0ULL;
+#endif
+
 template <class T> class numeric_limits {
 public:
   static constexpr T max();
   static constexpr T min();
 };
 
 // TODO: Add numeric_limits specializations as needed for new types.