llvm/llvm-project

Implement `clz` using floating point math if `clz` instruction is not available

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#161,746 opened on Oct 2, 2025

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good first issuellvm:SelectionDAGmissed-optimization

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Description

In https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogIEEE64Float we can find a neat trick to calculate the ilog2 of a u32 with floating point math:

u32 ilog2_u32(u32 x) {
    // zexto extend to 64 bits
    u64 x_u64 = x;

    // set exponent to 52
    x_u64 |= (52 + 1023ULL) << 52;

    // move into f64 register
    double f = __builtin_bit_cast(double, x_u64);

    // subtract 2^52
    f -= 0x1.0p52;

    // move back into u64 register
    x_u64 = __builtin_bit_cast(u64, f);

    // extract exponent
    return (x_u64 >> 52) - 1023;
}

Rearrange the equation:

ilog2(x) == 31 - clz(x)
clz(x) + ilog2(x) == 31
clz(x) == 31 - ilog2(x)

and we get:

u32 src32(u32 x) { return __builtin_clzg(x); }
u32 tgt32(u32 x) { return 31 - ilog2_u32(x); }

This generalizes to u8 and u16 using float instead of double

https://godbolt.org/z/cW8js3MK9 https://alive2.llvm.org/ce/z/-bK_nC

Contributor guide

Research direction
Evaluate the floating point ilog2 method for u8, u16, u32, u64 and integrate into LLVM's clz intrinsic lowering when hardware clz instruction is unavailable.
Tech stack
cpp
Domain
backend
Issue type
Feature
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3
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Half day
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Clear
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C++
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50