[PATCH] D12853: Introduce !value.align metadata for load instruction

[James Molloy via llvm-commits]

Hi,

I'm probably missing something obvious, but it sounds like you want to do
this:

%1 = load i8*, i8** %p, !align !0 ; %1 is pointer-to-value
%2 = load i8, i8* %1 ; value is %2

Why can't you instead mark all loads of %1 as aligned?:

%1 = load i8*, i8** %p, !align !0 ; %1 is pointer-to-value
%2 = load i8, i8* %1, align 4

Again, I'm probably missing something obvious.

Cheers,

James

On Mon, 14 Sep 2015 at 19:03 Artur Pilipenko via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> apilipenko created this revision.
> apilipenko added reviewers: hfinkel, reames, sanjoy.
> apilipenko added a subscriber: llvm-commits.
>
> Since D9791 we take alignment into account to decide if it's safe to
> speculate load. Now pointer has to be both dereferenceable and properly
> aligned to be speculative loadable. It means that we want to support align
> attribute everywhere we support dereferenceable attribute.
>
> Introduce new !value.align metadata for load instruction. This metadata
> specifies alignment of the loaded pointer value.
>
> http://reviews.llvm.org/D12853
>
> Files:
>   docs/LangRef.rst
>   include/llvm/IR/LLVMContext.h
>   lib/Analysis/ValueTracking.cpp
>   lib/IR/LLVMContext.cpp
>   test/Analysis/ValueTracking/memory-dereferenceable.ll
>
> Index: test/Analysis/ValueTracking/memory-dereferenceable.ll
> ===================================================================
> --- test/Analysis/ValueTracking/memory-dereferenceable.ll
> +++ test/Analysis/ValueTracking/memory-dereferenceable.ll
> @@ -111,6 +111,14 @@
>      %load21 = load i8, i8 addrspace(1)* %gep.align1.offset16, align 16
>      %load22 = load i8, i8 addrspace(1)* %gep.align16.offset16, align 16
>
> +    ; Load from a dereferenceable and aligned load
> +; CHECK: %d4_unaligned_load{{.*}}(unaligned)
> +; CHECK: %d4_aligned_load{{.*}}(aligned)
> +    %d4_unaligned_load = load i32*, i32** @globali32ptr, !dereferenceable
> !0
> +    %d4_aligned_load = load i32*, i32** @globali32ptr, !dereferenceable
> !0, !value.align !{i64 16}
> +    %load23 = load i32, i32* %d4_unaligned_load, align 16
> +    %load24 = load i32, i32* %d4_aligned_load, align 16
> +
>      ret void
>  }
>
> Index: lib/IR/LLVMContext.cpp
> ===================================================================
> --- lib/IR/LLVMContext.cpp
> +++ lib/IR/LLVMContext.cpp
> @@ -116,6 +116,12 @@
>    assert(UnpredictableID == MD_unpredictable &&
>           "unpredictable kind id drifted");
>    (void)UnpredictableID;
> +
> +  // Create the 'value.align' metadata kind.
> +  unsigned ValueAlignID = getMDKindID("value.align");
> +  assert(ValueAlignID == MD_value_align &&
> +         "value.align kind id drifted");
> +  (void)ValueAlignID;
>  }
>  LLVMContext::~LLVMContext() { delete pImpl; }
>
> Index: lib/Analysis/ValueTracking.cpp
> ===================================================================
> --- lib/Analysis/ValueTracking.cpp
> +++ lib/Analysis/ValueTracking.cpp
> @@ -2952,6 +2952,11 @@
>      BaseAlign = GV->getAlignment();
>    else if (const Argument *A = dyn_cast<Argument>(Base))
>      BaseAlign = A->getParamAlignment();
> +  else if (const LoadInst *LI = dyn_cast<LoadInst>(Base))
> +    if (MDNode *MD = LI->getMetadata(LLVMContext::MD_value_align)) {
> +      ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
> +      BaseAlign = CI->getLimitedValue();
> +    }
>
>    if (!BaseAlign) {
>      Type *Ty = Base->getType()->getPointerElementType();
> Index: include/llvm/IR/LLVMContext.h
> ===================================================================
> --- include/llvm/IR/LLVMContext.h
> +++ include/llvm/IR/LLVMContext.h
> @@ -62,7 +62,8 @@
>      MD_dereferenceable = 12, // "dereferenceable"
>      MD_dereferenceable_or_null = 13, // "dereferenceable_or_null"
>      MD_make_implicit = 14, // "make.implicit"
> -    MD_unpredictable = 15 // "unpredictable"
> +    MD_unpredictable = 15, // "unpredictable"
> +    MD_value_align = 16 // "value.align"
>    };
>
>    /// getMDKindID - Return a unique non-zero ID for the specified
> metadata kind.
> Index: docs/LangRef.rst
> ===================================================================
> --- docs/LangRef.rst
> +++ docs/LangRef.rst
> @@ -6768,9 +6768,10 @@
>
>  ::
>
> -      <result> = load [volatile] <ty>, <ty>* <pointer>[, align
> <alignment>][, !nontemporal !<index>][, !invariant.load !<index>][,
> !nonnull !<index>][, !dereferenceable !<index>][, !dereferenceable_or_null
> !<index>]
> +      <result> = load [volatile] <ty>, <ty>* <pointer>[, align
> <alignment>][, !nontemporal !<index>][, !invariant.load !<index>][,
> !nonnull !<index>][, !dereferenceable !<index>][, !dereferenceable_or_null
> !<index>][, !value.align !<value_align_node>]
>        <result> = load atomic [volatile] <ty>* <pointer> [singlethread]
> <ordering>, align <alignment>
>        !<index> = !{ i32 1 }
> +      !<value_align_node> = !{ i64 <alignment> }
>
>  Overview:
>  """""""""
> @@ -6852,6 +6853,14 @@
>  attribute on parameters and return values. This metadata can only be
> applied
>  to loads of a pointer type.
>
> +The optional ``!value.align`` metadata must reference a single
> +metadata name ``<value_align_node>`` corresponding to a metadata node
> with one
> +``i64`` entry. The existence of the ``!value.align`` metadata on the
> instruction
> +tells the optimizer that the value loaded is known to be aligned to a
> boundary
> +specified by the integer value in the metadata node. This is analogous to
> the
> +''align'' attribute on parameters and return values. This metadata can
> only be
> +applied to loads of a pointer type.
> +
>  Semantics:
>  """"""""""
>
>
>
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