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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213670 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 867ef741da17dac5f1f0ce9b3fd96f779dcf9ae4..34873e1f41d650602c76aa8c6f11f6926e59d218 100644 (file)
--- a/docs/LangRef.rst
+++ b/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/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index e6d09f4e31f6e19e17dafe4137ee35cabda8629a..4edfb615f2a03341122273ebb2ed2928cbf93f51 100644 (file)
--- a/lib/Analysis/ValueTracking.cpp
+++ b/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/test/Bitcode/attributes.ll b/test/Bitcode/attributes.ll
index 2490e59207260f20500d965bb7247d23081bc4b6..9a429f6e21f729bc3384b7bfab4ae0d247033777 100644 (file)
--- a/test/Bitcode/attributes.ll
+++ b/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/test/Transforms/InstCombine/align-attr.ll b/test/Transforms/InstCombine/align-attr.ll
new file mode 100644 (file)
index 0000000..9f366bf
--- /dev/null
+++ b/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
+}
+