There’s no single instruction that can do this. BMI1 blsi dst,src can isolate the lowest set bit, not the highest. i.e. x & -x. If x86 had a bit-reversed version of blsi, we could use that, but it doesn’t.

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But you can do much better than what you were suggesting. An all-zero input is always going to be a special case for bit-scan and shift. Otherwise our output has exactly 1 bit set. It’s 1 << bsr(input).

;; input: x in RDI
;; output: result in RAX
isolate_msb:
    xor   eax, eax           ; tmp = 0
    bsr   rdi, rdi           ; edi = bit index of MSB in input
    jz    .input_was_zero
    bts   rax, rdi           ; rax |= 1<<edi

.input_was_zero:             ; return 0 for input=0
    ret

Obviously for 32-bit inputs, use only 32-bit registers. And if zero is not possible, omit the JZ. Using BSR instead of LZCNT gives us a bit-index, not 31-bitidx, so we can use it directly. But LZCNT is significantly faster on AMD.

The xor-zeroing is off the critical path, to prepare an input for BTS. xor-zero + BTS is the most efficient way to implement 1<<n on Intel CPUs. It’s 2 uops with 2c latency on AMD, so mov rax,1 / shl rax,cl would be better there. But worse on Intel because variable-count shifts are 3 uops, unless you use BMI2 shlx.

Anyway, the real work here is BSR + BTS, so that’s 3 cycle + 1 cycle latency on Intel SnB-family. (https://agner.org/optimize/)

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In C / C++, you’d write this as

unsigned isolate_msb32(unsigned x) {
    unsigned bitidx = BSR32(x);
    //return 1ULL << bitidx;           // if x is definitely non-zero
    return x ? 1U << bitidx : x;
}

unsigned isolate_msb64(uint64_t x) {
    unsigned bitidx = BSR64(x);
    return x ? 1ULL << bitidx : x;
}

Where BSR32 is defined in terms of an intrinsic your compiler supports. This is where things get tricky, especially if you want a 64-bit version. There’s no single portable intrinsic. GNU C provides count-leading-zeros intrinsics, but GCC and ICC suck at optimizing 63-__builtin_clzll(x) back into just BSR. Instead they negate twice. There are builtins for BSR specifically, but those are even more compiler-specific than just MSVC vs. compilers that support GNU extensions (gcc/clang/ICC).

#include <stdint.h>

// define BSR32() and BSR64()
#if defined(_MSC_VER) || defined(__INTEL_COMPILER)
    #ifdef __INTEL_COMPILER
        typedef unsigned int bsr_idx_t;
    #else
        #include <intrin.h>   // MSVC
        typedef unsigned long bsr_idx_t;
    #endif

    static inline
    unsigned BSR32(unsigned long x){
        bsr_idx_t idx;
        _BitScanReverse(&idx, x); // ignore bool retval
        return idx;
    }
    static inline
    unsigned BSR64(uint64_t x) {
        bsr_idx_t idx;
        _BitScanReverse64(&idx, x); // ignore bool retval
        return idx;
    }
#elif defined(__GNUC__)

  #ifdef __clang__
    static inline unsigned BSR64(uint64_t x) {
        return 63-__builtin_clzll(x);
      // gcc/ICC can't optimize this back to just BSR, but clang can and doesn't provide alternate intrinsics
    }
  #else
    #define BSR64 __builtin_ia32_bsrdi
  #endif

    #include <x86intrin.h>
    #define BSR32(x) _bit_scan_reverse(x)

#endif

On the Godbolt compiler explorer, clang and ICC compile this branchlessly, even when they don’t know that x is non-zero.

All 4 compilers fail to use bts to implement 1<<bit. 🙁 It’s very cheap on Intel.

# clang7.0 -O3 -march=ivybridge   (for x86-64 System V)
# with -march=haswell and later it uses lzcnt and has to negate.  /sigh.
isolate_msb32(unsigned int):
        bsr     ecx, edi
        mov     eax, 1
        shl     rax, cl
        test    edi, edi
        cmove   eax, edi       # return 1<<bsr(x)  or  x (0) if x was zero
        ret

GCC and MSVC make branchy code. e.g.

# gcc8.2 -O3 -march=haswell
    mov     eax, edi
    test    edi, edi
    je      .L6
    bsr     eax, edi
    mov     edi, 1
    shlx    rax, rdi, rax    # BMI2:  1 uop instead of 3 for shl rax,cl
.L6:
    ret