Add a known bounds optimization to ASan.

test/Instrumentation/AddressSanitizer/OptKnownBounds/single-before-start.ll

Index: test/Instrumentation/AddressSanitizer/OptKnownBounds/single-before-start.ll
diff --git a/test/Instrumentation/AddressSanitizer/OptKnownBounds/single-before-start.ll b/test/Instrumentation/AddressSanitizer/OptKnownBounds/single-before-start.ll
new file mode 100644
index 0000000000000000000000000000000000000000..bb9b35936e06216f57b72966ee0dbafbc3cfe411
--- /dev/null
+++ b/test/Instrumentation/AddressSanitizer/OptKnownBounds/single-before-start.ll
@@ -0,0 +1,30 @@
+; Invalid access of the first element before the start of a global array.
+
+; RUN: opt < %s -asan -S -asan-opt=1 -asan-opt-known-bounds=1 | FileCheck %s -check-prefix=OPT1
+; RUN: opt < %s -asan -S -asan-opt=0 | FileCheck %s -check-prefix=OPT0
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@global_array = internal global [50 x i32] zeroinitializer, align 16
+
+define i32 @example() address_safety nounwind uwtable readonly {
+entry:
+  %0 = load i32* getelementptr inbounds ([50 x i32]* @global_array, i64 0, i64 50), align 4
+  ret i32 %0
+}
+
+; OPT1: @example
+; OPT1: call void @__asan_report_
+; OPT1-NOT: call void @__asan_report_
+; OPT1: @asan.module_ctor
+
+; OPT0: @example
+; OPT0: call void @__asan_report_
+; OPT0-NOT: call void @__asan_report_
+; OPT0: @asan.module_ctor
+
+; C-example: static int global_array[50];
+; C-example: int example(void) {
+; C-example:   return global_array[-1];  // BUG
+; C-example: }