[LLVMdev] [RFC] Stripping unusable intrinsics

[Chris Bieneman]

Ok… I bit.

Alex’s proposal here is really compelling to me because it means that the required changes to make this work would be more limited. Specifically a clang attribute could give us all the benefits of #ifdefs throughout the code without the maintenance burden. So, being the silly person I am, I wrote the patches for clang.

I’ve never done any frontend hacking before, so take these with giant cellars of salt, but the concept seems sound to me.

The patches do the following:
(1) Add a new C++11-style [[impossible_enum]] attribute
(2) Any case statement that has [[impossible_enum]] applied to it is not emitted during IRGen - the bodies are always emitted so as not to interfere with fall through, but blocks that cannot be entered are optimized away
(3) Equality comparison against [[impossible_enum]] values are always false, all other comparisons are always true

There was some discussion on IRC today whether or not this was the right way to do this, but I thought I’d send these patches out anyways so people can take a look.

The attached diffs are for clang, I’ve also attached a c++ test file.

-Chris

> On Dec 16, 2014, at 12:09 PM, Alex Rosenberg <alexr at ohmantics.com> wrote:
> 
> Random shower thought:
> 
> I think the markup can be minimized if it only appears once in the header where the enums are defined instead of at every place where the enums are used. Then we could value propagate that certain enum values are never possible where they're checked for. That should generally be able to strip the same set of stuff but use less markup.
> 
> Alex
> 
> On Dec 10, 2014, at 3:53 PM, Chris Bieneman <beanz at apple.com <mailto:beanz at apple.com>> wrote:
> 
>> llvm-dev,
>> 
>> In my ongoing saga to improve LLVM for embedded use, we would like to support stripping out unused intrinsics based on the LLVM targets actually being built.
>> 
>> I’ve attached two patches.
>> 
>> The first is a new flag for tablegen to take a list of targets. If passed tablegen will only emit intrinsics that either have empty target prefixes, or target prefixes matching one of the targets in the list. If the flag is not passed the behavior is unchanged. This patch can land today (subject to review).
>> 
>> The second patch is a WIP, and adds support to the CMake build system for using the new tablegen flag, and for generating a new llvm/Config/llvm-targets.h header which contains defines for each target specified with LLVM_TARGETS_TO_BUILD.
>> 
>> This new header will allow us to #ifdef code using target-specific intrinsics outside the targets, thus allowing us to strip out all the unused intrinsics.
>> 
>> -Chris
>> 
>> <cmake-build.diff><tablegen.diff>_______________________________________________
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