c++: strict aliasing

1 files changed, 203 insertions, 1 deletions

diff --git a/src/development/c++.md b/src/development/c++.md
index 2fc7a09..2df60c2 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -1,5 +1,7 @@
 # c++
 
+openstd [cpp standards][openstd-stds].
+
 Source files of most examples is available [here][src-examples].
 
 ## Type deduction
@@ -12,6 +14,199 @@ auto foo = bar();
 typename decltype(foo)::_;
 ```
 
+## Strict aliasing and type punning
+
+The `strict aliasing` rules describe via which `alias` a value can be accessed.
+> Informal: an `alias` is a reference / pointer to a value.
+
+Accessing a value through an alias that violates the strict aliasing rules is
+`undefined behavior (UB)`.
+
+Examples below on [godbolt](https://godbolt.org/z/TsvTY9zfj).
+```cpp
+int i = 0;
+
+// Valid aliasing (signed / unsigned type).
+*reinterpret_cast<signed int*>(&i);
+*reinterpret_cast<unsigned int*>(&i);
+
+// Valid aliasing (cv qualified type).
+*reinterpret_cast<const int*>(&i);
+*reinterpret_cast<const unsigned*>(&i);
+
+// Valid aliasing (byte type).
+*reinterpret_cast<char*>(&i);
+*reinterpret_cast<std::byte*>(&i);
+
+// Invalid aliasing, dereferencing pointer is UB.
+*reinterpret_cast<short*>(&i);
+*reinterpret_cast<float*>(&i);
+```
+> NOTE: Casting pointer to invalid aliasing type is not directly UB, but
+> dereferencing the pointer is UB.
+
+```cpp
+short s[2] = { 1, 2 };
+
+// Invalid aliasing (UB) - type punning, UB to deref ptr (int has stricter
+// alignment requirements than short).
+*reinterpret_cast<int*>(s);
+
+
+// Arbitrary byte pointer.
+char c[4] = { 1, 2, 3, 4 };
+
+// Invalid aliasing (UB) - type punning, UB to deref ptr (int has stricter
+// alignment requirements than char).
+*reinterpret_cast<int*>(c);
+```
+
+At the time of writing, the current [c++ std draft][std-draft-aliasing]
+contains the following.
+```text
+If a program attempts to access the stored value of an object through a glvalue
+whose type is not **similar** (7.3.6) to one of the following types the
+behavior is undefined [44]
+
+(11.1) the dynamic type of the object,
+(11.2) a type that is the signed or unsigned type corresponding to the dynamic
+       type of the object, or
+(11.3) a char, unsigned char, or std::byte type.
+
+[44]: The intent of this list is to specify those circumstances in which an
+      object can or cannot be aliased.
+```
+
+The paragraph is short but one also needs to understand the meaning of
+[similar (*similar_types*)][std-draft-similar-types].
+
+This paragraph is actually somewhat more explicit in the [c++17 std][std-17].
+```text
+If a program attempts to access the stored value of an object through a glvalue
+of other than one of the following types the behavior is undefined [63]
+
+(11.1) the dynamic type of the object,
+(11.2) a cv-qualified version of the dynamic type of the object,
+(11.3) a type similar (as defined in 7.5) to the dynamic type of the object,
+(11.4) a type that is the signed or unsigned type corresponding to the dynamic
+       type of the object,
+(11.5) a type that is the signed or unsigned type corresponding to a
+       cv-qualified version of the dynamic type of the object,
+(11.6) an aggregate or union type that includes one of the aforementioned types
+       among its elements or non- static data members (including, recursively,
+       an element or non-static data member of a subaggregate or contained
+       union),
+(11.7) a type that is a (possibly cv-qualified) base class type of the dynamic
+       type of the object,
+(11.8) a char, unsigned char, or std::byte type.
+
+[63]: The intent of this list is to specify those circumstances in which an
+      object may or may not be aliased.
+```
+
+Additional references:
+- [What is the Strict Aliasing Rule and Why do we care][gist-strict-aliasing]
+
+  The article shows a small example how the compiler may optimized using the
+  strict aliasing rules.
+  ```cpp
+  int alias(int* i, char* c) {
+    *i = 1;
+    *c = 'a';  // char* may alias int*
+    return *i;
+  }
+
+  int noalias(int* i, short* s) {
+      *i = 1;
+      *s = 2;  // short* does not alias int*
+      return *i;
+  }
+  ```
+  ```x86asm
+  alias(int*, char*):
+  mov    DWORD PTR [rdi] ,0x1  ; *i = 1;
+  mov    BYTE PTR [rsi], 0x61  ; *c = 'a';
+  mov    eax,DWORD PTR [rdi]   ; Must reload, char* can alias int*.
+  ret
+
+  noalias(int*, short*):
+  mov    DWORD PTR [rdi], 0x1  ; *i = 1;
+  mov    WORD PTR [rsi], 0x2   ; *s = 2;
+  mov    eax,0x1               ; Must not reload, short* can not alias int*.
+  ret
+  ```
+- [reinterpret_cast][reinterpret-aliasing] type aliasing
+  > 5) Any object pointer type `T1*` can be converted to another object pointer
+  > type `cv T2*`. This is exactly equivalent to `static_cast<cv
+  > T2*>(static_cast<cv void*>(expression))` (which implies that if T2's
+  > alignment requirement is not stricter than T1's, the value of the pointer
+  > does not change and conversion of the resulting pointer back to its
+  > original type yields the original value). In any case, the resulting
+  > pointer may only be dereferenced safely if allowed by the type aliasing
+  > rules (see below).
+
+  ```cpp
+  int I;
+  char* X = reinterpret_cast<char*>(&I);  // Valid, char allowed to alias int.
+  *X = 42;
+  int* Y = reinterpret_cast<int*>(X);     // Cast back to original type.
+  *Y = 1337;  // safe
+
+  char C[4];
+  int* P = reinterpret_cast<int*>(C);     // Cast is ok, not yet UB.
+  *P = 1337; // UB, violates strict aliasing / alignment rules.
+             // https://stackoverflow.com/questions/52492229/c-byte-array-to-int
+  ```
+ - On `gcc` strict aliasing is enabled starting with `-O2`.
+   ```bash
+   for i in {0..3} g s; do echo "-O$i $(g++ -Q --help=optimizers -O$i | grep fstrict-aliasing)"; done
+   -O0   -fstrict-aliasing           [disabled]
+   -O1   -fstrict-aliasing           [disabled]
+   -O2   -fstrict-aliasing           [enabled]
+   -O3   -fstrict-aliasing           [enabled]
+   -Og   -fstrict-aliasing           [disabled]
+   -Os   -fstrict-aliasing           [enabled]
+   ```
+
+### `__restrict` keyword
+
+The `__restrict` keyword allows the programmer to tell the compiler that two
+pointer will not alias each other.
+```cpp
+int alias(int* a, int* b) {
+    *a = 1;
+    *b = 2;
+    return *a;
+}
+
+// alias(int*, int*):                           # @alias(int*, int*)
+//         mov     dword ptr [rdi], 1
+//         mov     dword ptr [rsi], 2
+//         mov     eax, dword ptr [rdi]
+//         ret
+
+int noalias(int* __restrict a, int* __restrict b) {
+    *a = 1;
+    *b = 2;
+    return *a;
+}
+
+// noalias(int*, int*):                         # @noalias(int*, int*)
+//         mov     dword ptr [rdi], 1
+//         mov     dword ptr [rsi], 2
+//         mov     eax, 1
+//         ret
+```
+
+However this should only be used with care and in a narrow scope, as it is easy
+to violate self defined contract, see [godbolt](https://godbolt.org/z/e8x1af3Mh).
+
+### Type punning
+
+The correct way to do `type-punning` in c++:
+1. [`std::bit_cast`][std-bitcast] (c++20)
+1. [`std::memcpy`](https://godbolt.org/z/3PM4jGvEz)
+
 ## Variadic templates ([parameter pack][parameter-pack])
 
 ```cpp
@@ -98,10 +293,17 @@ A more detailed description is available in the SO discussion [How does
 {{#include c++/fwd-perfect.cc:3:}}
 ```
 
-[gist-strict-asliasing]: https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
 [parameter-pack]: https://en.cppreference.com/w/cpp/language/parameter_pack
 [enable-if]: https://en.cppreference.com/w/cpp/types/enable_if
 [sfinae]: https://en.cppreference.com/w/cpp/language/sfinae
 [fwd-ref]: https://en.cppreference.com/w/cpp/language/reference#Forwarding_references
 [std-fwd]: https://en.cppreference.com/w/cpp/utility/forward
+[std-bitcast]: https://en.cppreference.com/w/cpp/numeric/bit_cast
 [src-examples]: https://github.com/johannst/notes/tree/master/src/development/c%2B%2B
+[reinterpret-aliasing]: https://en.cppreference.com/w/cpp/language/reinterpret_cast#Type_aliasing
+[gist-strict-aliasing]: https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
+[std-draft-aliasing]: http://eel.is/c++draft/basic.lval#11
+[std-draft-similar-types]: http://eel.is/c++draft/conv.qual#def:similar_types
+[std-17]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4713.pdf
+[openstd-home]: https://www.open-std.org/jtc1/sc22/wg21/
+[openstd-stds]: https://www.open-std.org/jtc1/sc22/wg21/docs/standards