From c24d624c9224b95e57b346e767df7a067317f15f Mon Sep 17 00:00:00 2001 From: Kostya Serebryany Date: Mon, 11 Feb 2013 14:36:01 +0000 Subject: [PATCH] [asan] added a flag -mllvm asan-short-64bit-mapping-offset=1 (0 by default) This flag makes asan use a small (<2G) offset for 64-bit asan shadow mapping. On x86_64 this saves us a register, thus achieving ~2/3 of the zero-base-offset's benefits in both performance and code size. Thanks Jakub Jelinek for the idea. llvm-svn: 174886 --- lib/Transforms/Instrumentation/AddressSanitizer.cpp | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/lib/Transforms/Instrumentation/AddressSanitizer.cpp index 0474eb516aa..c5f77ec0893 100644 --- a/lib/Transforms/Instrumentation/AddressSanitizer.cpp +++ b/lib/Transforms/Instrumentation/AddressSanitizer.cpp @@ -53,6 +53,7 @@ using namespace llvm; static const uint64_t kDefaultShadowScale = 3; static const uint64_t kDefaultShadowOffset32 = 1ULL << 29; static const uint64_t kDefaultShadowOffset64 = 1ULL << 44; +static const uint64_t kDefaultShort64bitShadowOffset = 0x7FFF8000; // < 2G. static const uint64_t kPPC64_ShadowOffset64 = 1ULL << 41; static const size_t kMaxStackMallocSize = 1 << 16; // 64K @@ -133,6 +134,9 @@ static cl::opt ClMappingScale("asan-mapping-scale", cl::desc("scale of asan shadow mapping"), cl::Hidden, cl::init(0)); static cl::opt ClMappingOffsetLog("asan-mapping-offset-log", cl::desc("offset of asan shadow mapping"), cl::Hidden, cl::init(-1)); +static cl::opt ClShort64BitOffset("asan-short-64bit-mapping-offset", + cl::desc("Use short immediate constant as the mapping offset for 64bit"), + cl::Hidden, cl::init(false)); // Optimization flags. Not user visible, used mostly for testing // and benchmarking the tool. @@ -205,12 +209,14 @@ static ShadowMapping getShadowMapping(const Module &M, int LongSize, // OR-ing shadow offset if more efficient (at least on x86), // but on ppc64 we have to use add since the shadow offset is not neccesary // 1/8-th of the address space. - Mapping.OrShadowOffset = !IsPPC64; + Mapping.OrShadowOffset = !IsPPC64 && !ClShort64BitOffset; Mapping.Offset = (IsAndroid || ZeroBaseShadow) ? 0 : (LongSize == 32 ? kDefaultShadowOffset32 : IsPPC64 ? kPPC64_ShadowOffset64 : kDefaultShadowOffset64); - if (ClMappingOffsetLog >= 0) { + if (!ZeroBaseShadow && ClShort64BitOffset && LongSize == 64) { + Mapping.Offset = kDefaultShort64bitShadowOffset; + } if (!ZeroBaseShadow && ClMappingOffsetLog >= 0) { // Zero offset log is the special case. Mapping.Offset = (ClMappingOffsetLog == 0) ? 0 : 1ULL << ClMappingOffsetLog; }