[CodeGen] Use the zero initializer instead of storing an all zero representation.

Summary:
This change avoids the overhead of storing, and later crawling,
an initializer list of all zeros for arrays. When LLVM
visits this (llvm/IR/Constants.cpp) ConstantArray::getImpl()
it will scan the list looking for an array of all zero.

We can avoid the store, and short-cut the scan, by detecting
all zeros when clang builds-up the initialization representation.

This was brought to my attention when investigating PR36030


Reviewers: majnemer, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42549

llvm-svn: 324776

1 files changed, 12 insertions, 0 deletions

diff --git a/clang/test/CodeGen/array-init.c b/clang/test/CodeGen/array-init.c
new file mode 100644
index 00000000000..fa54994acc3
--- /dev/null
+++ b/clang/test/CodeGen/array-init.c
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 %s -O0 -triple x86_64-unknown-linux-gnu -emit-llvm -o - | FileCheck %s
+
+// CHECK: @{{.*}}.a1 = internal constant [5 x i32] [i32 0, i32 1, i32 2, i32 0, i32 0]
+// CHECK: @{{.*}}.a2 = internal constant [5 x i32] zeroinitializer
+// CHECK: @{{.*}}.a3 = internal constant [5 x i32] zeroinitializer
+
+void testConstArrayInits(void)
+{
+  const int a1[5] = {0,1,2};
+  const int a2[5] = {0,0,0};
+  const int a3[5] = {0};
+}