Make use of the align parameter attribute for all pointer arguments

We previously supported the align attribute on all (pointer) parameters, but we
only used it for byval parameters. However, it is completely consistent at the
IR level to treat 'align n' on all pointer parameters as an alignment
assumption on the pointer, and now we wll. Specifically, this causes
computeKnownBits to use the align attribute on all pointer parameters, not just
byval parameters. I've also added an explicit parameter attribute test for this
to test/Bitcode/attributes.ll.

And I've updated the LangRef to document the align parameter attribute (as it
turns out, it was not documented at all previously, although the byval
documentation mentioned that it could be used).

There are (at least) two benefits to doing this:
 - It allows enhancing alignment based on the pointer alignment after inlining callees.
 - It allows simplification of pointer arithmetic.

llvm-svn: 213670

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 867ef74..34873e1 100644 (file)
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -921,6 +921,13 @@ Currently, only the following parameter attributes are defined:
     the first parameter. This is not a valid attribute for return
     values.
 
+``align <n>``
+    This indicates that the pointer value may be assumed by the optimizer to
+    have the specified alignment.
+
+    Note that this attribute has additional semantics when combined with the
+    ``byval`` attribute.
+
 .. _noalias:
 
 ``noalias``

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e6d09f4..4edfb61 100644 (file)
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -308,13 +308,9 @@ void llvm::computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
   }
 
   if (Argument *A = dyn_cast<Argument>(V)) {
-    unsigned Align = 0;
+    unsigned Align = A->getType()->isPointerTy() ? A->getParamAlignment() : 0;
 
-    if (A->hasByValOrInAllocaAttr()) {
-      // Get alignment information off byval/inalloca arguments if specified in
-      // the IR.
-      Align = A->getParamAlignment();
-    } else if (TD && A->hasStructRetAttr()) {
+    if (!Align && TD && A->hasStructRetAttr()) {
       // An sret parameter has at least the ABI alignment of the return type.
       Type *EltTy = cast<PointerType>(A->getType())->getElementType();
       if (EltTy->isSized())

diff --git a/llvm/test/Bitcode/attributes.ll b/llvm/test/Bitcode/attributes.ll
index 2490e59..9a429f6 100644 (file)
--- a/llvm/test/Bitcode/attributes.ll
+++ b/llvm/test/Bitcode/attributes.ll
@@ -239,6 +239,11 @@ define dereferenceable(18446744073709551606) i8* @f40(i8* dereferenceable(184467
         ret i8* %a
 }
 
+define void @f41(i8* align 32, double* align 64) {
+; CHECK: define void @f41(i8* align 32, double* align 64) {
+        ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }

diff --git a/llvm/test/Transforms/InstCombine/align-attr.ll b/llvm/test/Transforms/InstCombine/align-attr.ll
new file mode 100644 (file)
index 0000000..9f366bf
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/align-attr.ll
@@ -0,0 +1,15 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define i32 @foo1(i32* align 32 %a) #0 {
+entry:
+  %0 = load i32* %a, align 4
+  ret i32 %0
+
+; CHECK-LABEL: @foo1
+; CHECK-DAG: load i32* %a, align 32
+; CHECK: ret i32
+}
+