[clang] [llvm] [HLSL] implement the rcp intrinsic (PR #83857)

[Farzon Lotfi via cfe-commits]

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+// RUN: %clang_cc1 -finclude-default-header -triple dxil-pc-shadermodel6.6-library %s -fnative-half-type -emit-llvm -disable-llvm-passes -verify -verify-ignore-unexpected
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farzonl wrote:

not a bad idea, it shares the same sema checks as frac will try and get back to you.

https://github.com/llvm/llvm-project/pull/83857