RyuJIT should be able to discover and emit bool-to-int conversions in the most efficient way.

[redknightlois]

Usual ways to convert boolean into integers in C# are:

• If true then 1 else 0
• Convert.ToInt32(bool)
• Unsafe bool in the stacks tricks.

While the latest is the most efficient, it is a pain to use. The other two which are used in plenty places including the Framework itself could emit a very efficient assembly code instead of the standard method call (as of right now).

Differences in performance are huge:

// BenchmarkDotNet=v0.7.6.0
// OS=Microsoft Windows NT 6.2.9200.0
// Processor=Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz, ProcessorCount=4
// CLR=MS.NET 4.0.30319.42000, Arch=64-bit [RyuJIT]
Common: Type=Jit_BoolToInt Mode=Throughput Platform=X64 .NET=Current

Method	Jit	AvrTime	StdDev	op/s
Framework	LegacyJit	1.16 us	28.20 ns	858517.79
IfThenElse	LegacyJit	1.08 us	44.58 ns	929458.93
UnsafeConvert	LegacyJit	595.83 ns	5.79 ns	1678333.61
Framework	RyuJit	1.31 us	18.64 ns	765539.56
IfThenElse	RyuJit	1.31 us	16.05 ns	761023.73
UnsafeConvert	RyuJit	896.80 ns	6.27 ns	1115078.04

Repro: https://gist.github.com/redknightlois/c1ae5ddc6f73c2e53c9b

category:cq
theme:optimization
skill-level:intermediate
cost:small

[BruceForstall]

You could do the "unsafe" way even faster (and safer; the repro case looks illegal to me):

        public int UnsafeConvert2()
        {
            int sum = 0;
            byte[] ab = Unsafe.As<byte[]>(bits);
            for (int i = 0; i < N; i++)
            {
                sum += ab[i];
            }
            return sum;
        }

Method	Mean	Error	StdDev	Code Size
Framework	1,181.6 ns	3.20 ns	2.99 ns	70 B
IfThenElse	1,192.6 ns	12.21 ns	10.82 ns	67 B
UnsafeConvert	607.3 ns	3.84 ns	3.59 ns	74 B
UnsafeConvert2	419.9 ns	1.47 ns	1.23 ns	54 B

The problem is this is unsafe because the spec says:

III.1.1.2 Boolean data type
A CLI Boolean type occupies 1 byte in memory. A bit pattern of all zeroes denotes a value of
false. A bit pattern with any one or more bits set (analogous to a non-zero integer) denotes a
value of true. For the purpose of stack operations boolean values are treated as unsigned 1-byte
integers (§III.1.1.1).

There may be processor-specific implementations that could generate better code for the first two cases (which are equivalent, actually). Given that this optimization is very specific, and requires a specific pattern, unless there is a real scenario where this is important, it's unlikely to be implemented. Thus, I'm going to close this.

[BruceForstall]

Note that if-conversion is covered in #6749 and https://github.com/dotnet/runtime/blob/master/docs/design/coreclr/jit/JitOptimizerTodoAssessment.md