[llvm-dev] rotl: undocumented LLVM instruction?

[Hans Wennborg via llvm-dev]

On Wed, Nov 2, 2016 at 3:24 PM, Phil Tomson via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> We've recently moved our project from LLVM 3.6 to LLVM 3.9.  I noticed  one
> of our code generation tests is breaking in 3.9.
>
> The test is:
>
>  ; RUN: llc < %s -march=xstg | FileCheck %s
>
> define i64 @bclr64(i64 %a, i64 %b) nounwind readnone {
> entry:
> ; CHECK: bclr     r1, r0, r1, 64
>   %sub = sub i64 %b, 1
>   %shl = shl i64 1, %sub
>   %xor = xor i64 %shl, -1
>   %and = and i64 %a, %xor
>   ret i64 %and
> }
>
> I ran llc with -debug to get a better idea of what's going on and found:
>
> Initial selection DAG: BB#0 'bclr64:entry'
> SelectionDAG has 14 nodes:
>   t0: ch = EntryToken
>       t2: i64,ch = CopyFromReg t0, Register:i64 %vreg0
>             t4: i64,ch = CopyFromReg t0, Register:i64 %vreg1
>           t6: i64 = sub t4, Constant:i64<1>
>         t7: i64 = shl Constant:i64<1>, t6
>       t9: i64 = xor t7, Constant:i64<-1>
>     t10: i64 = and t2, t9
>   t12: ch,glue = CopyToReg t0, Register:i64 %R1, t10
>   t13: ch = XSTGISD::Ret t12, Register:i64 %R1, t12:1
>
>
>
> Combining: t13: ch = XSTGISD::Ret t12, Register:i64 %R1, t12:1
>
> Combining: t12: ch,glue = CopyToReg t0, Register:i64 %R1, t10
>
> Combining: t11: i64 = Register %R1
>
> Combining: t10: i64 = and t2, t9
>
> Combining: t9: i64 = xor t7, Constant:i64<-1>
>  ... into: t15: i64 = rotl Constant:i64<-2>, t6
>
> Combining: t10: i64 = and t2, t15
>
> Combining: t15: i64 = rotl Constant:i64<-2>, t6
>
> Combining: t14: i64 = Constant<-2>
>
> Combining: t6: i64 = sub t4, Constant:i64<1>
>  ... into: t17: i64 = add t4, Constant:i64<-1>
>
> Combining: t15: i64 = rotl Constant:i64<-2>, t17
>
>
>
> These rotl instructions weren't showing up when I ran llc 3.6 and that's
> completely changing the generated code at the end which means the test fails
> (and it's less optimal than it was in 3.6).
>
> I've been looking in the LLVM language docs (3.9 version) and I don't see
> any documentation on 'rotl'. What does it do? Why isn't it in the docs?

rotl is not an IR instruction, it's a node in the
instruction-selection dag (ISD::ROTL). It performs bitwise rotation to
the left.

The change to your code is probably due to this new transformation:
http://llvm.org/viewvc/llvm-project?view=revision&revision=232572