Preserve param alignment in NVPTXLowerArgs pass.

NVPTXLowerArgs works as follows.

  * Create a regular alloca with alignment identical to arg.
  * Copy arg from param space (and ASC'ing it from generic AS first) to
    the alloca (it's still in generic AS).
  * Replace loads of arg with loads of alloca.

The bug here is that we did not preserve the arg's alignment when
loading from the alloca.

The impact of this bug is that sometimes param loads would be lowered as
a series of u8 loads, because we're incorrectly assuming everything has
alignment 1.

Differential Revision: https://reviews.llvm.org/D89404

diff --git a/llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp b/llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
index d685933..fd58ff1 100644 (file)
--- a/llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
@@ -172,8 +172,12 @@ void NVPTXLowerArgs::handleByValParam(Argument *Arg) {
   Value *ArgInParam = new AddrSpaceCastInst(
       Arg, PointerType::get(StructType, ADDRESS_SPACE_PARAM), Arg->getName(),
       FirstInst);
+  // Be sure to propagate alignment to this load; LLVM doesn't know that NVPTX
+  // addrspacecast preserves alignment.  Since params are constant, this load is
+  // definitely not volatile.
   LoadInst *LI =
-      new LoadInst(StructType, ArgInParam, Arg->getName(), FirstInst);
+      new LoadInst(StructType, ArgInParam, Arg->getName(),
+                   /*isVolatile=*/false, AllocA->getAlign(), FirstInst);
   new StoreInst(LI, AllocA, FirstInst);
 }
 

diff --git a/llvm/test/CodeGen/NVPTX/lower-args.ll b/llvm/test/CodeGen/NVPTX/lower-args.ll
new file mode 100644 (file)
index 0000000..fbd97b9
--- /dev/null
+++ b/llvm/test/CodeGen/NVPTX/lower-args.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -S -nvptx-lower-args | FileCheck %s --check-prefix IR
+; RUN: llc < %s -mcpu=sm_20 | FileCheck %s --check-prefix PTX
+
+target datalayout = "e-i64:64-i128:128-v16:16-v32:32-n16:32:64"
+target triple = "nvptx64-nvidia-cuda"
+
+%class.outer = type <{ %class.inner, i32, [4 x i8] }>
+%class.inner = type { i32*, i32* }
+
+; Check that nvptx-lower-args preserves arg alignment
+define void @load_alignment(%class.outer* nocapture readonly byval(%class.outer) align 8 %arg) {
+entry:
+; IR: load %class.outer, %class.outer addrspace(101)*
+; IR-SAME: align 8
+; PTX: ld.param.u64
+; PTX-NOT: ld.param.u8
+  %arg.idx = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 0, i32 0
+  %arg.idx.val = load i32*, i32** %arg.idx, align 8
+  %arg.idx1 = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 0, i32 1
+  %arg.idx1.val = load i32*, i32** %arg.idx1, align 8
+  %arg.idx2 = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 1
+  %arg.idx2.val = load i32, i32* %arg.idx2, align 8
+  %arg.idx.val.val = load i32, i32* %arg.idx.val, align 4
+  %add.i = add nsw i32 %arg.idx.val.val, %arg.idx2.val
+  store i32 %add.i, i32* %arg.idx1.val, align 4
+  ret void
+}