This patch adds a predicateinfo intrinsic, as part of splitting up D29316.

[• dberlin]

Summary

The intrinsic, marked as returning it's first argument, has no code
generation effect (though currently not every optimization pass knows
that intrinsics with the returned attribute can be looked through).

It is used to attach predicate information to existing operands, and
be able to tell what the predicate information affects.

We deliberately do not attach any info through a second operand so
that the intrinsics do not need to dominate the comparisons/etc (since
in the case of assume, we may want to push them up the post-dominator
tree).

[• dberlin]

• Add PredicateInfo utility, and printing pass.

[• dberlin]

Sigh, one sec, it screwed this up

[• dberlin]

Fixing update

[sanjoy]

docs/LangRef.rst

12689

How about calling this llvm.ssa_copy or something like that, given how general this is?

12702

"attached" is kind of vague -- can you be more specific here?

[• dberlin]

docs/LangRef.rst

12689

I kind of deliberately gave it a name that made it so people wouldn't try to use it as a generic copy intrinsic, because i'm not sure we want that used elsewhere :)

But if we do, ...

12702

Sure, would "the first argument is an operand which the PredicateInfo utility creates information about. Transforms/Utils/PredicateInfo.h has more information, but a brief overview is below".

work?

IE how specific do you want it?

[• dberlin]

• Update for review comments

[davide]

The intrinsic description (and the intrinsic itself, LGTM). I'm reviewing predicate info next.

docs/LangRef.rst

 used by the ``llvm.assume`` intrinsic in order to preserve the instructions
 only used to form the intrinsic's input argument. This might prove undesirable
 if the extra information provided by the ``llvm.assume`` intrinsic does not cause
 sufficient overall improvement in code quality. For this reason,
 ``llvm.assume`` should not be used to document basic mathematical invariants
 that the optimizer can otherwise deduce or facts that are of little use to the
 optimizer.
 
+.. _int_ssa_copy:
+
+'``llvm.ssa_copy``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare type @llvm.ssa_copy(type %operand) returned(1) readnone
+
+Arguments:
+""""""""""
+
+The first argument is an operand which is used as the returned value.
+
+Overview:
+""""""""""
+
+The ``llvm.ssa_copy`` intrinsic can be used to attach information to
+operations by copying them and giving them new names.  For example,
+the PredicateInfo utility uses it to build Extended SSA form, and
+attach various forms of information to operands that dominate specific
+uses.  It is not meant for general use, only for building temporary
+renaming forms that require value splits at certain points.
+
 .. _type.test:
 
 '``llvm.type.test``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""
 

include/llvm/IR/Intrinsics.td

 //
 
 def int_memcpy_element_atomic  : Intrinsic<[],
                                            [llvm_anyptr_ty, llvm_anyptr_ty,
                                             llvm_i64_ty, llvm_i32_ty],
                                  [IntrArgMemOnly, NoCapture<0>, NoCapture<1>,
                                   WriteOnly<0>, ReadOnly<1>]>;
 
+//===----- Intrinsics that are used to provide predicate information -----===//
+
+def int_ssa_copy : Intrinsic<[llvm_any_ty], [LLVMMatchType<0>],
+           [IntrNoMem, Returned<0>]>;
 //===----------------------------------------------------------------------===//
 // Target-specific intrinsics
 //===----------------------------------------------------------------------===//
 
 include "llvm/IR/IntrinsicsPowerPC.td"
 include "llvm/IR/IntrinsicsX86.td"
 include "llvm/IR/IntrinsicsARM.td"
 include "llvm/IR/IntrinsicsAArch64.td"
 include "llvm/IR/IntrinsicsXCore.td"
 include "llvm/IR/IntrinsicsHexagon.td"
 include "llvm/IR/IntrinsicsNVVM.td"
 include "llvm/IR/IntrinsicsMips.td"
 include "llvm/IR/IntrinsicsAMDGPU.td"
 include "llvm/IR/IntrinsicsBPF.td"
 include "llvm/IR/IntrinsicsSystemZ.td"
 include "llvm/IR/IntrinsicsWebAssembly.td"