From c03496d4d02ee1d560a80c323e5bbef7b3bd7320 Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor@mac.com>
Date: Sun, 8 Mar 2015 21:53:59 +0000
Subject: [PATCH] Teach DataLayout to infer a plausible alignment for things
 even when nothing is specified by the user.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231613 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/IR/DataLayout.cpp                           | 17 ++++++++++++++---
 .../Transforms/InstCombine/default-alignment.ll | 10 ++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 test/Transforms/InstCombine/default-alignment.ll

diff --git a/lib/IR/DataLayout.cpp b/lib/IR/DataLayout.cpp
index c70d7c68a91..5dcb5fbf490 100644
--- a/lib/IR/DataLayout.cpp
+++ b/lib/IR/DataLayout.cpp
@@ -479,9 +479,7 @@ unsigned DataLayout::getAlignmentInfo(AlignTypeEnum AlignType,
     // If we didn't find an integer alignment, fall back on most conservative.
     if (AlignType == INTEGER_ALIGN) {
       BestMatchIdx = LargestInt;
-    } else {
-      assert(AlignType == VECTOR_ALIGN && "Unknown alignment type!");
-
+    } else if (AlignType == VECTOR_ALIGN) {
       // By default, use natural alignment for vector types. This is consistent
       // with what clang and llvm-gcc do.
       unsigned Align = getTypeAllocSize(cast<VectorType>(Ty)->getElementType());
@@ -494,6 +492,19 @@ unsigned DataLayout::getAlignmentInfo(AlignTypeEnum AlignType,
     }
   }
 
+  // If we still couldn't find a reasonable default alignment, fall back
+  // to a simple heuristic that the alignment is the first power of two
+  // greater-or-equal to the store size of the type.  This is a reasonable
+  // approximation of reality, and if the user wanted something less
+  // less conservative, they should have specified it explicitly in the data
+  // layout.
+  if (BestMatchIdx == -1) {
+    unsigned Align = getTypeStoreSize(Ty);
+    if (Align & (Align-1))
+      Align = NextPowerOf2(Align);
+    return Align;
+  }
+
   // Since we got a "best match" index, just return it.
   return ABIInfo ? Alignments[BestMatchIdx].ABIAlign
                  : Alignments[BestMatchIdx].PrefAlign;
diff --git a/test/Transforms/InstCombine/default-alignment.ll b/test/Transforms/InstCombine/default-alignment.ll
new file mode 100644
index 00000000000..718da213671
--- /dev/null
+++ b/test/Transforms/InstCombine/default-alignment.ll
@@ -0,0 +1,10 @@
+; RUN: opt -verify -instcombine < %s
+%Foo = type <{ i8, x86_fp80 }>
+
+define i8 @t(%Foo* %arg) {
+entry:
+  %0 = getelementptr %Foo, %Foo* %arg, i32 0, i32 0
+  %1 = load i8, i8* %0, align 1
+  ret i8 %1
+}
+