Inefficient codegen with ternary and additive inverse

While reviewing #133659, I had a gut feeling about:

int M1(int num, bool r) => num + (r ? 0 : -1);
// vs.
int M2(int num, bool r) => num - (r ? 0 : 1);

https://godbolt.org/z/3czc6z7f9

Expected:

mov  eax, edi
cmp  sil, 1
sbb  eax, 0
ret

@JulieLeeMSFT

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I don't see how that is valid codegen for this IL btw - bool can be any value 0-255, and that codegen is only correct for 0-1 - 2-255 bool is UB in C# but not in ECMA-335 - you would have to write C# like num - 1 + (r ? 1 : 0) and change roslyn to get it to emit IL more like:

ldarg.0
ldc.i4.1
sub
ldarg.1
add
ret

(that is, extend the bool ? 1 : 0 -> (int)bool optimization to any bool, not just for direct IL comparison sequences like x >= 0 ? 1 : 0).

As you can see, it works as expected if I insert the cast using Unsafe.BitCast instead: https://godbolt.org/z/78PMe5cPd

Program:M1(int,bool):int (FullOpts):
       movzx    rax, sil
       lea      eax, [rdi+rax-0x01]
       ret

https://github.com/dotnet/roslyn/blob/main/docs/compilers/Boolean%20Representation.md

@hamarb123

@hamarb123, good point, updated expected to cover 2+

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Inefficient codegen with ternary and additive inverse #135647

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Inefficient codegen with ternary and additive inverse #135647

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dotnet-policy-service commented on Oct 12, 2026

hamarb123 commented on Oct 13, 2026

hamarb123 commented on Oct 13, 2026

am11 commented on Oct 13, 2026

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