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

[Artur Pilipenko via llvm-commits]

apilipenko updated this revision to Diff 34895.
apilipenko added a comment.

Add a sentence to LangRef to require alignment to be a power of 2.


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. The alignment must be a
+power of 2. 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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