How to get single-operand imul from C code

Question

What do I have to write in C to get the assembler to show the imul with one operand? For example:

imul  %ebp

Answer 1

If you want to write C code so that the compiler emits imul with one operand then the only way is to use widening signed multiplication, i.e. cast the operand(s) to a signed type twice the register length. Hence in 32-bit mode the following code will produce the expected output

long long multiply(int x, int y) {
    return (long long)x * y;
}

Because non-widening multiplication in 2's complement is the same for both signed and unsigned types, compilers will almost always use multi-operand imul as it's faster and more flexible.

In x86_64 compilers use multi-operand imul to produce 64-bit results even when the inputs were only 32-bits because they still typically have better latency and throughput than single-operand imul. As a result you'll also have to use a double-register-width (128-bit) type like above. Clang, ICC and GCC do support that via __int128, though.

• What gcc versions support the __int128 intrinsic type?
• Is there a 128 bit integer in gcc?

The below snippet

__int128_t multiply(long long x, long long y) {
    return  (__int128_t)x * y;
}

will be compiled to

mov     rax, rdi
imul    rsi
ret

If you use MSVC then you can use _umul128 in 64-bit mode