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Reland "simplify SkTFitsIn, try 2"

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This is a reland of 009f51e43f

Original change's description:
> simplify SkTFitsIn, try 2
> 
> Originally we wrote this in C++14 constexpr,
> but because this is used from public headers, we can't use
> C++14 yet.  Now a somewhat sillier looking C++11 version.
> 
> All the old tests still pass, and one new added.
> Updated the comments a bit for correctness and readability.
> 
> Change-Id: I99c01e1346c1a5a36278cc08f30538112e5259aa
> Reviewed-on: https://skia-review.googlesource.com/134425
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> Commit-Queue: Ben Wagner <bungeman@google.com>
> Auto-Submit: Mike Klein <mtklein@chromium.org>
> Reviewed-by: Ben Wagner <bungeman@google.com>

Change-Id: I99676faac6153830538c1396a325ca1456dfb126
Reviewed-on: https://skia-review.googlesource.com/134800
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
This commit is contained in:
Mike Klein 2018-06-11 11:56:57 -04:00 committed by Skia Commit-Bot
parent bdde428d53
commit b6b5b7a808
2 changed files with 31 additions and 166 deletions
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195
include/private/SkTFitsIn.h
View File

@ -8,7 +8,7 @@
#ifndef SkTFitsIn_DEFINED
#define SkTFitsIn_DEFINED
#include "../private/SkTLogic.h"
#include <cassert>
#include <limits>
#include <type_traits>
@ -25,15 +25,20 @@
* (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
*
* Consider the cases:
* u = unsigned, s = signed, X = more digits, x = less digits
* ux -> uX: (ux)(uX)ux == ux, trivially true
* uX -> ux: (uX)(ux)uX == uX, both casts well defined, test works
* sx -> sX: (sx)(sX)sx == sx, trivially true
* sX -> sx: (sX)(sx)sX == sX, first cast implementation value, second cast defined, test works
* sx -> uX: (sx)(uX)sx == sx, this is bad, the second cast results in implementation defined value
* sX -> ux: (sX)(ux)sX == sX, the second cast is required to prevent promotion of rhs to unsigned
* ux -> sX: (ux)(sX)ux == ux, trivially true
* uX -> sx: (uX)(sx)uX == uX, this is bad,
* u = unsigned, less digits
* U = unsigned, more digits
* s = signed, less digits
* S = signed, more digits
* v is the value we're considering.
*
* u -> U: (u)(U)v == v, trivially true
* U -> u: (U)(u)v == v, both casts well defined, test works
* s -> S: (s)(S)v == v, trivially true
* S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works
* s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value
* S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned
* u -> S: (u)(S)v == v, trivially true
* U -> s: (U)(s)v == v, *this is bad*,
* first cast results in implementation defined value,
* second cast is defined. However, this creates false positives
* uint16_t x = 0xFFFF
@ -44,167 +49,25 @@
*
* So for the eight cases three are trivially true, three more are valid casts, and two are special.
* The two 'full' checks which otherwise require two comparisons are valid cast checks.
* The two remaining checks uX -> sx [uX < max(sx)] and sx -> uX [sx > 0] can be done with one op.
* The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op.
*/
namespace sktfitsin {
namespace Private {
template <typename D, typename S>
static constexpr inline bool SkTFitsIn(S src) {
// SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11.
return assert(std::is_integral<S>::value || std::is_enum<S>::value),
assert(std::is_integral<D>::value || std::is_enum<D>::value),
/** SkTMux::type = (a && b) ? Both : (a) ? A : (b) ? B : Neither; */
template <bool a, bool b, typename Both, typename A, typename B, typename Neither>
struct SkTMux {
using type = skstd::conditional_t<a, skstd::conditional_t<b, Both, A>,
skstd::conditional_t<b, B, Neither>>;
};
// E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1.
(std::is_signed<S>::value && std::is_unsigned<D>::value && sizeof(S) <= sizeof(D)) ?
(S)0 <= src :
/** SkTHasMoreDigits = (digits(A) >= digits(B)) ? true_type : false_type. */
template <typename A, typename B> struct SkTHasMoreDigits
: skstd::bool_constant<std::numeric_limits<A>::digits >= std::numeric_limits<B>::digits>
{ };
// E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1.
(std::is_signed<D>::value && std::is_unsigned<S>::value && sizeof(D) <= sizeof(S)) ?
src <= (S)std::numeric_limits<D>::max() :
/** Returns true.
* Used when it is statically known that source values are in the range of the Destination.
*/
template <typename S> struct SkTInRange_True {
static constexpr bool fits(S) {
return true;
}
};
/** Tests that (S)(D)s == s.
* This is not valid for uX -> sx and sx -> uX conversions.
*/
template <typename D, typename S> struct SkTInRange_Cast {
static constexpr bool fits(S s) {
using S_is_bigger = SkTHasMoreDigits<S, D>;
using D_is_bigger = SkTHasMoreDigits<D, S>;
using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;
using precondition = skstd::bool_constant<
!((!S_is_signed::value && D_is_signed::value && S_is_bigger::value) ||
( S_is_signed::value && !D_is_signed::value && D_is_bigger::value) )>;
static_assert(precondition::value, "not valid for uX -> sx and sx -> uX conversions");
return static_cast<S>(static_cast<D>(s)) == s;
}
};
/** Tests if the source value <= Max(D).
* Assumes that Max(S) >= Max(D).
*/
template <typename D, typename S> struct SkTInRange_LE_MaxD {
static constexpr bool fits(S s) {
using precondition = SkTHasMoreDigits<S, D>;
static_assert(precondition::value, "maxS < maxD");
return s <= static_cast<S>((std::numeric_limits<D>::max)());
}
};
/** Tests if the source value >= 0. */
template <typename D, typename S> struct SkTInRange_GE_Zero {
static constexpr bool fits(S s) {
return static_cast<S>(0) <= s;
}
};
/** SkTFitsIn_Unsigned2Unsiged::type is an SkTInRange with an fits(S s) method
* the implementation of which is tailored for the source and destination types.
* Assumes that S and D are unsigned integer types.
*/
template <typename D, typename S> struct SkTFitsIn_Unsigned2Unsiged {
using CastCheck = SkTInRange_Cast<D, S>;
using NoCheck = SkTInRange_True<S>;
// If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
};
/** SkTFitsIn_Signed2Signed::type is an SkTInRange with an fits(S s) method
* the implementation of which is tailored for the source and destination types.
* Assumes that S and D are signed integer types.
*/
template <typename D, typename S> struct SkTFitsIn_Signed2Signed {
using CastCheck = SkTInRange_Cast<D, S>;
using NoCheck = SkTInRange_True<S>;
// If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
};
/** SkTFitsIn_Signed2Unsigned::type is an SkTInRange with an fits(S s) method
* the implementation of which is tailored for the source and destination types.
* Assumes that S is a signed integer type and D is an unsigned integer type.
*/
template <typename D, typename S> struct SkTFitsIn_Signed2Unsigned {
using CastCheck = SkTInRange_Cast<D, S>;
using LowSideOnlyCheck = SkTInRange_GE_Zero<D, S>;
// If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(),
// no need to check the high side. (Until C++11, assume more digits means greater max.)
// This also protects the precondition of SkTInRange_Cast.
using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
using type = skstd::conditional_t<sourceCannotExceedDest::value, LowSideOnlyCheck, CastCheck>;
};
/** SkTFitsIn_Unsigned2Signed::type is an SkTInRange with an fits(S s) method
* the implementation of which is tailored for the source and destination types.
* Assumes that S is an usigned integer type and D is a signed integer type.
*/
template <typename D, typename S> struct SkTFitsIn_Unsigned2Signed {
using HighSideCheck = SkTInRange_LE_MaxD<D, S>;
using NoCheck = SkTInRange_True<S>;
// If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(), nothing to check.
// (Until C++11, assume more digits means greater max.)
using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
using type = skstd::conditional_t<sourceCannotExceedDest::value, NoCheck, HighSideCheck>;
};
/** SkTFitsIn::type is an SkTInRange with an fits(S s) method
* the implementation of which is tailored for the source and destination types.
* Assumes that S and D are integer types.
*/
template <typename D, typename S> struct SkTFitsIn {
// One of the following will be the 'selector' type.
using S2S = SkTFitsIn_Signed2Signed<D, S>;
using S2U = SkTFitsIn_Signed2Unsigned<D, S>;
using U2S = SkTFitsIn_Unsigned2Signed<D, S>;
using U2U = SkTFitsIn_Unsigned2Unsiged<D, S>;
using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;
using selector = typename SkTMux<S_is_signed::value, D_is_signed::value,
S2S, S2U, U2S, U2U>::type;
// This type is an SkTInRange.
using type = typename selector::type;
};
template <typename T, bool = std::is_enum<T>::value> struct underlying_type {
using type = skstd::underlying_type_t<T>;
};
template <typename T> struct underlying_type<T, false> {
using type = T;
};
} // namespace Private
} // namespace sktfitsin
/** Returns true if the integer source value 's' will fit in the integer destination type 'D'. */
template <typename D, typename S> constexpr inline bool SkTFitsIn(S s) {
static_assert(std::is_integral<S>::value || std::is_enum<S>::value, "S must be integral.");
static_assert(std::is_integral<D>::value || std::is_enum<D>::value, "D must be integral.");
using RealS = typename sktfitsin::Private::underlying_type<S>::type;
using RealD = typename sktfitsin::Private::underlying_type<D>::type;
return sktfitsin::Private::SkTFitsIn<RealD, RealS>::type::fits(static_cast<RealS>(s));
// else
(S)(D)src == src;
}
#endif

2
tests/FitsInTest.cpp
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@ -14,6 +14,8 @@
#define TEST(S, s, D, expected) REPORTER_ASSERT(reporter, (SkTFitsIn<D>((S)(s)) == (expected)))
DEF_TEST(FitsIn, reporter) {
TEST(uint16_t, 257, int8_t, false);
TEST(int32_t, 1, int8_t, true);
TEST(int32_t, -1, int8_t, true);
TEST(int32_t, (int32_t)(std::numeric_limits<int8_t>::max)(), int8_t, true);