[LLVMdev] Adding an intrinsic that requires a constant parameter

[Rafael Espíndola]

The shufflevector instruction has this property on the mask, not sure
if the same implementation would work for an intrinsic.

On 1 May 2014 08:10, Justin Holewinski <justin.holewinski at gmail.com> wrote:
> My understanding of intrinsics in LLVM is that an optimization pass cannot
> touch an intrinsic unless it (1) is specifically programmed to have
> knowledge of the intrinsic, or (2) uses only declared properties of the
> intrinsic, like IntrNoMem.  So, for example, a pass may move an unknown
> intrinsic around memory ops if its defined IntrNoMem, but it cannot hoist
> constants for arbitrary intrinsics.  Is this correct?  I can't find this
> codified anywhere in the documentation.
>
> The context for this question is adding an intrinsic that requires a
> constant parameter.  There is no way to define this property in the tablegen
> description currently.  I want to verify that if I add such an intrinsic, it
> would be illegal for any existing optimization pass to hoist the constant
> parameter.  For example, if I add a target intrinsic llvm.foo that takes two
> parameters: i32 and i1, where the i1 parameter must be a constant (same as
> llvm.cttz), I want to verify that the following optimization is illegal
> without having to change any LLVM passes:
>
> Input:
>
> define i32 @f0(i32 %a, i1 %pred) {
>   br i1 %pred, label %first, label %second
> first:
>   %r0 = tail call i32 @llvm.foo.i32(i32 %a, i1 true)
>   br label %end
> second:
>   %r1 = tail call i32 @llvm.foo.i32(i32 %a, i1 false)
>   br label %end
> end:
>   %r = phi i32 [%r0, %first], [%r1, %second]
>   ret i32 %r
> }
>
> Output:
>
> define i32 @f1(i32 %a, i1 %pred) {
>   %r = tail call i32 @llvm.foo.i32(i32 %a, i1 %pred)
>   ret i32 %r
> }
>
> --
>
> Thanks,
>
> Justin Holewinski
>
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