[asan] Prevent rematerialization of &__asan_shadow.

Summary:
In the mode when ASan shadow base is computed as the address of an
external global (__asan_shadow, currently on android/arm32 only),
regalloc prefers to rematerialize this value to save register spills.
Even in -Os. On arm32 it is rather expensive (2 loads + 1 constant
pool entry).

This changes adds an inline asm in the function prologue to suppress
this behavior. It reduces AsanTest binary size by 7%.

Reviewers: pcc, vitalybuka

Subscribers: aemerson, kristof.beyls, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40048

llvm-svn: 318235

1 files changed, 30 insertions, 12 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index d2e15ee649d..9681de4e885 100644
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -211,7 +211,13 @@ static cl::opt<bool>
     ClWithIfunc("asan-with-ifunc",
                 cl::desc("Access dynamic shadow through an ifunc global on "
                          "platforms that support this"),
-                cl::Hidden, cl::init(false));
+                cl::Hidden, cl::init(true));
+
+static cl::opt<bool> ClWithIfuncSuppressRemat(
+    "asan-with-ifunc-suppress-remat",
+    cl::desc("Suppress rematerialization of dynamic shadow address by passing "
+             "it through inline asm in prologue."),
+    cl::Hidden, cl::init(true));
 
 // This flag limits the number of instructions to be instrumented
 // in any given BB. Normally, this should be set to unlimited (INT_MAX),
@@ -988,8 +994,9 @@ struct FunctionStackPoisoner : public InstVisitor<FunctionStackPoisoner> {
   void visitCallSite(CallSite CS) {
     Instruction *I = CS.getInstruction();
     if (CallInst *CI = dyn_cast<CallInst>(I)) {
-      HasNonEmptyInlineAsm |=
-          CI->isInlineAsm() && !CI->isIdenticalTo(EmptyInlineAsm.get());
+      HasNonEmptyInlineAsm |= CI->isInlineAsm() &&
+                              !CI->isIdenticalTo(EmptyInlineAsm.get()) &&
+                              I != ASan.LocalDynamicShadow;
       HasReturnsTwiceCall |= CI->canReturnTwice();
     }
   }
@@ -1121,11 +1128,6 @@ Value *AddressSanitizer::memToShadow(Value *Shadow, IRBuilder<> &IRB) {
   if (Mapping.Offset == 0) return Shadow;
   // (Shadow >> scale) | offset
   Value *ShadowBase;
-  if (Mapping.InGlobal)
-    return IRB.CreatePtrToInt(
-        IRB.CreateGEP(AsanShadowGlobal,
-                      {ConstantInt::get(IntptrTy, 0), Shadow}),
-        IntptrTy);
   if (LocalDynamicShadow)
     ShadowBase = LocalDynamicShadow;
   else
@@ -2343,13 +2345,29 @@ bool AddressSanitizer::maybeInsertAsanInitAtFunctionEntry(Function &F) {
 
 void AddressSanitizer::maybeInsertDynamicShadowAtFunctionEntry(Function &F) {
   // Generate code only when dynamic addressing is needed.
-  if (Mapping.Offset != kDynamicShadowSentinel || Mapping.InGlobal)
+  if (Mapping.Offset != kDynamicShadowSentinel)
     return;
 
   IRBuilder<> IRB(&F.front().front());
-  Value *GlobalDynamicAddress = F.getParent()->getOrInsertGlobal(
-      kAsanShadowMemoryDynamicAddress, IntptrTy);
-  LocalDynamicShadow = IRB.CreateLoad(GlobalDynamicAddress);
+  if (Mapping.InGlobal) {
+    if (ClWithIfuncSuppressRemat) {
+      // An empty inline asm with input reg == output reg.
+      // An opaque pointer-to-int cast, basically.
+      InlineAsm *Asm = InlineAsm::get(
+          FunctionType::get(IntptrTy, {AsanShadowGlobal->getType()}, false),
+          StringRef(""), StringRef("=r,0"),
+          /*hasSideEffects=*/false);
+      LocalDynamicShadow =
+          IRB.CreateCall(Asm, {AsanShadowGlobal}, ".asan.shadow");
+    } else {
+      LocalDynamicShadow =
+          IRB.CreatePointerCast(AsanShadowGlobal, IntptrTy, ".asan.shadow");
+    }
+  } else {
+    Value *GlobalDynamicAddress = F.getParent()->getOrInsertGlobal(
+        kAsanShadowMemoryDynamicAddress, IntptrTy);
+    LocalDynamicShadow = IRB.CreateLoad(GlobalDynamicAddress);
+  }
 }
 
 void AddressSanitizer::markEscapedLocalAllocas(Function &F) {