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[asan] added a flag -mllvm asan-short-64bit-mapping-offset=1 (0 by default)

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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
This commit is contained in:
Kostya Serebryany 2013-02-11 14:36:01 +00:00
parent f2c4616277
commit c24d624c92
1 changed files with 8 additions and 2 deletions
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10
lib/Transforms/Instrumentation/AddressSanitizer.cpp
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@ -53,6 +53,7 @@ using namespace llvm;
static const uint64_t kDefaultShadowScale = 3; static const uint64_t kDefaultShadowScale = 3;
static const uint64_t kDefaultShadowOffset32 = 1ULL << 29; static const uint64_t kDefaultShadowOffset32 = 1ULL << 29;
static const uint64_t kDefaultShadowOffset64 = 1ULL << 44; static const uint64_t kDefaultShadowOffset64 = 1ULL << 44;
static const uint64_t kDefaultShort64bitShadowOffset = 0x7FFF8000; // < 2G.
static const uint64_t kPPC64_ShadowOffset64 = 1ULL << 41; static const uint64_t kPPC64_ShadowOffset64 = 1ULL << 41;
static const size_t kMaxStackMallocSize = 1 << 16; // 64K static const size_t kMaxStackMallocSize = 1 << 16; // 64K
@ -133,6 +134,9 @@ static cl::opt<int> ClMappingScale("asan-mapping-scale",
cl::desc("scale of asan shadow mapping"), cl::Hidden, cl::init(0)); cl::desc("scale of asan shadow mapping"), cl::Hidden, cl::init(0));
static cl::opt<int> ClMappingOffsetLog("asan-mapping-offset-log", static cl::opt<int> ClMappingOffsetLog("asan-mapping-offset-log",
cl::desc("offset of asan shadow mapping"), cl::Hidden, cl::init(-1)); cl::desc("offset of asan shadow mapping"), cl::Hidden, cl::init(-1));
static cl::opt<bool> 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 // Optimization flags. Not user visible, used mostly for testing
// and benchmarking the tool. // 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), // 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 // but on ppc64 we have to use add since the shadow offset is not neccesary
// 1/8-th of the address space. // 1/8-th of the address space.
Mapping.OrShadowOffset = !IsPPC64; Mapping.OrShadowOffset = !IsPPC64 && !ClShort64BitOffset;
Mapping.Offset = (IsAndroid || ZeroBaseShadow) ? 0 : Mapping.Offset = (IsAndroid || ZeroBaseShadow) ? 0 :
(LongSize == 32 ? kDefaultShadowOffset32 : (LongSize == 32 ? kDefaultShadowOffset32 :
IsPPC64 ? kPPC64_ShadowOffset64 : kDefaultShadowOffset64); 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. // Zero offset log is the special case.
Mapping.Offset = (ClMappingOffsetLog == 0) ? 0 : 1ULL << ClMappingOffsetLog; Mapping.Offset = (ClMappingOffsetLog == 0) ? 0 : 1ULL << ClMappingOffsetLog;
} }