[wasm-simd] Fix loading fp pair registers

ngzhian · Commit Bot · commit 8c698702ced0 · 2021-01-08T15:03:02.000Z
We were incorrectly clearing the high reg from the list of regs to load. The intention was to prevent double (and incorrect) loading - loading 128 bits from the low fp and the loading 128 bits from the high fp. But this violates the assumption that the two regs in a pair would be set or unset at the same time. The fix here is to introduce a new enum for register loads, a nop, which does nothing. The high fp of the fp pair will be tied to this nop, so as we iterate down the reglist, we load 128 bits using the low fp, then don't load anything for the high fp. Bug: chromium:1161654 Change-Id: If2ea79132b78623e5990237c60cf0883d9a8223f Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617380 Reviewed-by: Clemens Backes <clemensb@chromium.org> Commit-Queue: Zhi An Ng <zhin@chromium.org> Cr-Commit-Position: refs/heads/master@{#71976}
diff --git a/src/wasm/baseline/liftoff-assembler.cc b/src/wasm/baseline/liftoff-assembler.cc
@@ -38,6 +38,7 @@ class StackTransferRecipe {
 
   struct RegisterLoad {
     enum LoadKind : uint8_t {
+      kNop,           // no-op, used for high fp of a fp pair.
       kConstant,      // load a constant value into a register.
       kStack,         // fill a register from a stack slot.
       kLowHalfStack,  // fill a register from the low half of a stack slot.
@@ -64,6 +65,10 @@ class StackTransferRecipe {
       return {half == kLowWord ? kLowHalfStack : kHighHalfStack, kWasmI32,
               offset};
     }
+    static RegisterLoad Nop() {
+      // ValueType does not matter.
+      return {kNop, kWasmI32, 0};
+    }
 
    private:
     RegisterLoad(LoadKind kind, ValueType type, int32_t value)
@@ -220,11 +225,11 @@ class StackTransferRecipe {
           RegisterLoad::HalfStack(stack_offset, kHighWord);
     } else if (dst.is_fp_pair()) {
       DCHECK_EQ(kWasmS128, type);
-      // load_dst_regs_.set above will set both low and high fp regs.
-      // But unlike gp_pair, we load a kWasm128 in one go in ExecuteLoads.
-      // So unset the top fp register to skip loading it.
-      load_dst_regs_.clear(dst.high());
+      // Only need register_load for low_gp since we load 128 bits at one go.
+      // Both low and high need to be set in load_dst_regs_ but when iterating
+      // over it, both low and high will be cleared, so we won't load twice.
       *register_load(dst.low()) = RegisterLoad::Stack(stack_offset, type);
+      *register_load(dst.high()) = RegisterLoad::Nop();
     } else {
       *register_load(dst) = RegisterLoad::Stack(stack_offset, type);
     }
@@ -321,6 +326,8 @@ class StackTransferRecipe {
     for (LiftoffRegister dst : load_dst_regs_) {
       RegisterLoad* load = register_load(dst);
       switch (load->kind) {
+        case RegisterLoad::kNop:
+          break;
         case RegisterLoad::kConstant:
           asm_->LoadConstant(dst, load->type == kWasmI64
                                       ? WasmValue(int64_t{load->value})
diff --git a/test/mjsunit/regress/wasm/regress-1161654.js b/test/mjsunit/regress/wasm/regress-1161654.js
@@ -0,0 +1,56 @@
+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --wasm-staging
+
+// This is a fuzzer-generated test case that exposed a bug in Liftoff that only
+// affects ARM, where the fp register aliasing is different from other archs.
+// We were inncorrectly clearing the the high fp register in a LiftoffRegList
+// indicating registers to load, hitting a DCHECK.
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+const builder = new WasmModuleBuilder();
+builder.addMemory(19, 32, false);
+builder.addGlobal(kWasmI32, 0);
+builder.addType(makeSig([], []));
+builder.addType(makeSig([kWasmI64, kWasmS128, kWasmF32], [kWasmI32]));
+// Generate function 1 (out of 5).
+builder.addFunction(undefined, 0 /* sig */)
+  .addBodyWithEnd([
+// signature: v_v
+// body:
+kExprI32Const, 0x05,  // i32.const
+kExprReturn,  // return
+kExprUnreachable,  // unreachable
+kExprEnd,  // end @5
+]);
+// Generate function 4 (out of 5).
+builder.addFunction(undefined, 1 /* sig */)
+  .addBodyWithEnd([
+// signature: i_lsf
+// body:
+kExprLocalGet, 0x01,  // local.get
+kExprLocalGet, 0x01,  // local.get
+kExprGlobalGet, 0x00,  // global.get
+kExprDrop,  // drop
+kExprLoop, kWasmStmt,  // loop @8
+  kExprLoop, 0x00,  // loop @10
+    kExprI32Const, 0x01,  // i32.const
+    kExprMemoryGrow, 0x00,  // memory.grow
+    kExprI64LoadMem8U, 0x00, 0x70,  // i64.load8_u
+    kExprLoop, 0x00,  // loop @19
+      kExprCallFunction, 0x00,  // call function #0: v_v
+      kExprEnd,  // end @23
+    kExprI64Const, 0xf1, 0x24,  // i64.const
+    kExprGlobalGet, 0x00,  // global.get
+    kExprDrop,  // drop
+    kExprBr, 0x00,  // br depth=0
+    kExprEnd,  // end @32
+  kExprEnd,  // end @33
+kExprI32Const, 0x5b,  // i32.const
+kExprReturn,  // return
+kExprEnd,  // end @37
+]);
+// Instantiation is enough to cause a crash.
+const instance = builder.instantiate();
