Questions tagged [assumption]
6 questions
13
votes
2 answers
What happens when an assumption, i.e. [[assume]] contains UB?
In C++23, the [[assume(expression)]] attribute makes it so that if expression is false, the behavior is undefined.
For example:
int div(int x, int y) {
[[assume(y == 1)]];
return x / y;
}
This compiles to the same code as if y was always…
Jan Schultke
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6
votes
1 answer
Why is [[assume]] not evaluated but also potentially evaluated?
The cppreference page for [[assume]] says that:
[[assume( expression )]]
[...]
the expression is not evaluated (but it is still potentially evaluated).
This wording confuses me. Is cppreference wrong here?
Why would it be potentially evaluated…
Jan Schultke
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5
votes
3 answers
Reproducing clang's __builtin_assume for GCC
Recently, I discovered void __builtin_assume(bool) for clang, which can provide additional information about the state of the program to the compiler. This can make a huge difference, like for example:
#include
// compiles to about 80…
Jan Schultke
- 17,446
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- 96
1
vote
2 answers
What happens if an assumption, i.e. [[assume]] fails in a constant expression?
In C++23, the [[assume(conditonal-expression)]] attribute makes it so that if conditional-expression doesn't evaluate to true, the behavior is undefined.
For example:
int div(int x, int y) {
[[assume(y == 1)]];
return x / y;
}
This compiles…
Jan Schultke
- 17,446
- 6
- 47
- 96
1
vote
0 answers
Is it possible to make MSVC's __assume(0) aka std::unreachable() actually optimize?
Compiling the following code with MSVC
#include
#include
static auto bit_width(unsigned long x) {
unsigned long i;
_BitScanReverse(&i, x);
++i;
if (x == 0) { i = 0; }
return i;
}
auto foo(unsigned long x)…
user541686
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