Bug 1455610: Prevent nop fills from happening in jump tables; r=lth

--HG--
extra : rebase_source : 7597099d50710e0b095a06e064a76404f88e4ecc
extra : amend_source : 0b97b9f26872b921410a91d229c5ea6d3e76ac58

js/src/jit-test/tests/wasm/regress/baseline-nops-jumptable.js

@@ -0,0 +1,12 @@
+// |jit-test| --arm-asm-nop-fill=1
+var f = wasmEvalText(`(module (func (result i32) (param i32)
+      (block $0
+       (block $1
+        (block $2
+         (block $default
+          (br_table $0 $1 $2 $default (get_local 0))))))
+      (return (i32.const 0)))
+    (export "" 0)
+)`).exports[""];
+
+f(0);

js/src/jit/arm/Assembler-arm.cpp

@@ -2756,6 +2756,18 @@ Assembler::leaveNoPool()
     m_buffer.leaveNoPool();
 }
 
+void
+Assembler::enterNoNops()
+{
+    m_buffer.enterNoNops();
+}
+
+void
+Assembler::leaveNoNops()
+{
+    m_buffer.leaveNoNops();
+}
+
 ptrdiff_t
 Assembler::GetBranchOffset(const Instruction* i_)
 {

js/src/jit/arm/Assembler-arm.h

@@ -1958,6 +1958,8 @@ class Assembler : public AssemblerShared
     void flushBuffer();
     void enterNoPool(size_t maxInst);
     void leaveNoPool();
+    void enterNoNops();
+    void leaveNoNops();
     // This should return a BOffImm, but we didn't want to require everyplace
     // that used the AssemblerBuffer to make that class.
     static ptrdiff_t GetBranchOffset(const Instruction* i);
@@ -2487,6 +2489,19 @@ class AutoForbidPools
     }
 };
 
+// Forbids nop filling for testing purposes. Not nestable.
+class AutoForbidNops
+{
+    Assembler* masm_;
+  public:
+    explicit AutoForbidNops(Assembler* masm) : masm_(masm) {
+        masm_->enterNoNops();
+    }
+    ~AutoForbidNops() {
+        masm_->leaveNoNops();
+    }
+};
+
 } // namespace jit
 } // namespace js
 

js/src/jit/arm64/Assembler-arm64.h

@@ -521,19 +521,32 @@ void PatchJump(CodeLocationJump& jump_, CodeLocationLabel label);
 class AutoForbidPools
 {
     Assembler* asm_;
-
   public:
     AutoForbidPools(Assembler* asm_, size_t maxInst)
       : asm_(asm_)
     {
         asm_->enterNoPool(maxInst);
     }
-
     ~AutoForbidPools() {
         asm_->leaveNoPool();
     }
 };
 
+// Forbids nop filling for testing purposes. Not nestable.
+class AutoForbidNops
+{
+    Assembler* asm_;
+  public:
+    explicit AutoForbidNops(Assembler* asm_)
+      : asm_(asm_)
+    {
+        asm_->enterNoNops();
+    }
+    ~AutoForbidNops() {
+        asm_->leaveNoNops();
+    }
+};
+
 } // namespace jit
 } // namespace js
 

js/src/jit/arm64/vixl/MozBaseAssembler-vixl.h

@@ -286,6 +286,13 @@ class MozBaseAssembler : public js::jit::AssemblerShared {
     armbuffer_.leaveNoPool();
   }
 
+  void enterNoNops() {
+    armbuffer_.enterNoNops();
+  }
+  void leaveNoNops() {
+    armbuffer_.leaveNoNops();
+  }
+
  public:
   // Static interface used by IonAssemblerBufferWithConstantPools.
   static void InsertIndexIntoTag(uint8_t* load, uint32_t index);

js/src/jit/shared/IonAssemblerBufferWithConstantPools.h

@@ -642,6 +642,7 @@ struct AssemblerBufferWithConstantPools : public AssemblerBuffer<SliceSize, Inst
     // followed.
     const uint32_t nopFillInst_;
     const unsigned nopFill_;
+
     // For inhibiting the insertion of fill NOPs in the dynamic context in which
     // they are being inserted.
     bool inhibitNops_;
@@ -1083,7 +1084,17 @@ struct AssemblerBufferWithConstantPools : public AssemblerBuffer<SliceSize, Inst
         canNotPlacePool_ = false;
 
         // Validate the maxInst argument supplied to enterNoPool().
-        MOZ_ASSERT(this->nextOffset().getOffset() - canNotPlacePoolStartOffset_ <= canNotPlacePoolMaxInst_ * InstSize);
+        MOZ_ASSERT(this->nextOffset().getOffset() - canNotPlacePoolStartOffset_ <=
+                   canNotPlacePoolMaxInst_ * InstSize);
+    }
+
+    void enterNoNops() {
+        MOZ_ASSERT(!inhibitNops_);
+        inhibitNops_ = true;
+    }
+    void leaveNoNops() {
+        MOZ_ASSERT(inhibitNops_);
+        inhibitNops_ = false;
     }
 
     void align(unsigned alignment) {

js/src/wasm/WasmBaselineCompile.cpp

@@ -3835,6 +3835,10 @@ class BaseCompiler final : public BaseCompilerInterface
         // constant pool entries.
         masm.flush();
 
+#if defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_ARM64)
+        // Prevent nop sequences to appear in the jump table.
+        AutoForbidNops afn(&masm);
+#endif
         masm.bind(theTable);
 
         for (uint32_t i = 0; i < labels.length(); i++) {