[asan] add option to set shadow mapping offset

Allowing overriding the default ASAN shadow mapping offset with the
-asan-shadow-offset option, and allow zero to be specified for both offset and
scale.

Patch by Aaron Carroll <aaronc@apple.com>.

llvm-svn: 268724

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index f2b5179b436..635e0894d94 100644
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -204,10 +204,13 @@ static cl::opt<bool> ClSkipPromotableAllocas(
 
 // These flags allow to change the shadow mapping.
 // The shadow mapping looks like
-//    Shadow = (Mem >> scale) + (1 << offset_log)
+//    Shadow = (Mem >> scale) + offset
 static cl::opt<int> ClMappingScale("asan-mapping-scale",
                                    cl::desc("scale of asan shadow mapping"),
                                    cl::Hidden, cl::init(0));
+static cl::opt<unsigned long long> ClMappingOffset("asan-mapping-offset",
+                                   cl::desc("offset of asan shadow mapping [EXPERIMENTAL]"),
+                                   cl::Hidden, cl::init(0));
 
 // Optimization flags. Not user visible, used mostly for testing
 // and benchmarking the tool.
@@ -404,10 +407,14 @@ static ShadowMapping getShadowMapping(Triple &TargetTriple, int LongSize,
   }
 
   Mapping.Scale = kDefaultShadowScale;
-  if (ClMappingScale) {
+  if (ClMappingScale.getNumOccurrences() > 0) {
     Mapping.Scale = ClMappingScale;
   }
 
+  if (ClMappingOffset.getNumOccurrences() > 0) {
+    Mapping.Offset = ClMappingOffset;
+  }
+
   // OR-ing shadow offset if more efficient (at least on x86) if the offset
   // is a power of two, but on ppc64 we have to use add since the shadow
   // offset is not necessary 1/8-th of the address space.  On SystemZ,