# Failure with \`lshr\_int\` (intrinsic for \>\>) on UInt256 type

**URL:** <https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738>\
**Category:** General Usage\
**Created:** [October 4, 2020, 2:55pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738 "2020-10-04T14:55:47Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [October 4, 2020, 2:55pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/1 "2020-10-04T14:55:47Z")

</div>

Basically `lshr_int` works as expected when called directly, but not when wrapped in a function. I need to understand better the failure to find a proper fix in [BitIntegers](https://github.com/rfourquet/BitIntegers.jl). Here is a demo:

```julia
julia> using BitIntegers; x = rand(UInt256)
0xbe970fced92b8fcef396a1f5b40c63a86071b8f0330f43290f1d5d748262565f

julia> x >> 4
0x0be970fced92b8fcef396a1f5b40c63a86071b8f0330f43290f1d5d748262565

julia> x >> 0x4
0x0000000000000000000000000000000000000000000000000000000000000000

julia> x >> 0x0004
0x0be970fced92b8fcef396a1f5b40c63a86071b8f0330f43290f1d5d748262565

julia> Base.lshr_int(x, 0x4)
0x0be970fced92b8fcef396a1f5b40c63a86071b8f0330f43290f1d5d748262565

julia> f(x, y) = Base.lshr_int(x, y); f(x, 0x04)
0x0000000000000000000000000000000000000000000000000000000000000000

```

To what extent is it “safe” to use these intrinsics, and is it possible to change Julia to make the example above work?

---

<div class="post-metadata">

**Author:** ![simeonschaub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simeonschaub/32/216566_2.png) [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)\
**Post date:** [October 4, 2020, 3:10pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/2 "2020-10-04T15:10:11Z")

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Interestingly, on nightly, I get this:

```julia
julia> using BitIntegers; x = rand(UInt256)
[Info: Precompiling BitIntegers [c3b6d118-76ef-56ca-8cc7-ebb389d030a1]
0x00000000000000000000000000000000000000000000000000000000000000a1

```

So everything `rand` generates, except for the last 8 bits somehow is always zero.

Edit: It’s not just `rand`:

```julia
julia> uint256"0xbe970fced92b8fcef396a1f5b40c63a86071b8f0330f43290f1d5d748262565f"
0x000000000000000000000000000000000000000000000000000000000000005f

```

---

<div class="post-metadata">

**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [October 4, 2020, 3:19pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/3 "2020-10-04T15:19:49Z")

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Actually what you observe is a consequence of the OP, and it’s how I was made aware of it from [https://github.com/JuliaLang/julia/pull/36470#issuecomment-703036745:](https://github.com/JuliaLang/julia/pull/36470#issuecomment-703036745:)

```julia
125/126> rand(UInt256)
0x00000000000000000000000000000000000000000000000000000000000000d2

127> string(big(ans), base=16)
"28fecc917ccc6d1bbf9b54c500c26d0c8dea1d23520b8d4d87e9377ddf6099d2"

```

---

<div class="post-metadata">

**Author:** ![simeonschaub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simeonschaub/32/216566_2.png) [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)\
**Post date:** [October 4, 2020, 3:24pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/4 "2020-10-04T15:24:52Z")

</div>

It looks like it’s an LLVM bug:

```julia
julia> primitive type UInt256 256 end

julia> x = reinterpret(UInt256, rand(UInt128, 2))[]
UInt256(0x48805fdfb72d71fadaf8daa563cf816383f755d9d9e70d703fa348f556c717ad)

julia> f(x) = Base.lshr_int(x, 0x8)
f (generic function with 1 method)

julia> @code_llvm optimize=false f(x)
; @ REPL[22]:1 within `f'
define void @julia_f_392(i256* noalias nocapture sret %0, i256 %1) {
top:
  %2 = call {}*** @julia.ptls_states()
  %3 = bitcast {} ***%2 to {}**
  %4 = getelementptr inbounds {}*, {}** %3, i64 4
  %5 = bitcast {} **%4 to i64**
  %6 = load i64*, i64** %5
  %7 = lshr i256 %1, 8
  %8 = select i1 true, i256 0, i256 %7
  store i256 %8, i256* %0, align 8
  ret void
}

julia> @code_llvm optimize=true f(x)
; @ REPL[22]:1 within `f'
define void @julia_f_394(i256* noalias nocapture sret %0, i256 %1) {
top:
  store i256 0, i256* %0, align 8
  ret void
}

```

Would you mind opening an issue on GitHub?

---

<div class="post-metadata">

**Author:** ![simeonschaub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simeonschaub/32/216566_2.png) [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)\
**Post date:** [October 4, 2020, 3:27pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/5 "2020-10-04T15:27:33Z")

</div>

Ah, so it just prints wrong

---

<div class="post-metadata">

**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [October 4, 2020, 4:17pm UTC](https://discourse.julialang.org/t/failure-with-lshr-int-intrinsic-for-on-uint256-type/47738/6 "2020-10-04T16:17:51Z")

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> [@simeonschaub](#):
>
> Would you mind opening an issue on GitHub?

Ah great, thank you! Issue opened [Codegen bug with Base.lshr\_int intrinsic · Issue #37880 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/37880).

> [@simeonschaub](#):
>
> Ah, so it just prints wrong

Yes, sorry that I was not more clear.