[PATCH] Use align parameter attribute for all pointer arguments

[Chandler Carruth]

LGTM, seems like obvious goodness to me.
On Jul 22, 2014 9:04 AM, "hfinkel at anl.gov" <hfinkel at anl.gov> wrote:

> Hi chandlerc, reames, aschwaighofer, nadav,
>
> This is almost more of a documentation change than anything else...
>
> We currently support the align attribute on all (pointer) parameters, but
> we only use 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. This patch does this:
>
>  1. Causes computeKnownBits to use the align attribute on all pointer
> parameters, not just byval parameters.
>  2. 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 two benefits to doing this:
>  1. It allows enhancing alignment based on the pointer alignment after
> inlining
>  2. It allows simplification of pointer arithmetic
>
> My primary use case for these things involves pointers that are
> over-aligned to allow for efficient vectorized code generation.
>
> Thanks again!
>
> http://reviews.llvm.org/D4620
>
> Files:
>   docs/LangRef.rst
>   lib/Analysis/ValueTracking.cpp
>   test/Bitcode/attributes.ll
>   test/Transforms/InstCombine/align-attr.ll
>
> Index: docs/LangRef.rst
> ===================================================================
> --- docs/LangRef.rst
> +++ docs/LangRef.rst
> @@ -921,6 +921,13 @@
>      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``
> Index: lib/Analysis/ValueTracking.cpp
> ===================================================================
> --- lib/Analysis/ValueTracking.cpp
> +++ lib/Analysis/ValueTracking.cpp
> @@ -308,13 +308,9 @@
>    }
>
>    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())
> Index: test/Bitcode/attributes.ll
> ===================================================================
> --- test/Bitcode/attributes.ll
> +++ test/Bitcode/attributes.ll
> @@ -239,6 +239,11 @@
>          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 }
> Index: test/Transforms/InstCombine/align-attr.ll
> ===================================================================
> --- /dev/null
> +++ 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
> +}
> +
>
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