[wasm] Detect unrepresentability in the float32-to-int32 conversion correctly on arm.

src/compiler/arm/code-generator-arm.cc

 // Copyright 2014 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.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/arm/macro-assembler-arm.h"
 #include "src/ast/scopes.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
@@ skipping 1103 matching lines @@
       SwVfpRegister scratch = kScratchDoubleReg.low();
       __ vmov(scratch, i.InputRegister(0));
       __ vcvt_f64_u32(i.OutputDoubleRegister(), scratch);
       DCHECK_EQ(LeaveCC, i.OutputSBit());
       break;
     }
     case kArmVcvtS32F32: {
       SwVfpRegister scratch = kScratchDoubleReg.low();
       __ vcvt_s32_f32(scratch, i.InputFloatRegister(0));
       __ vmov(i.OutputRegister(), scratch);
+      // Avoid INT32_MAX as an overflow indicator and use INT32_MIN instead,
+      // because INT32_MIN allows easier out-of-bounds detection.
+      __ cmn(i.OutputRegister(), Operand(1));
+      __ add(i.OutputRegister(), i.OutputRegister(), Operand(1), SBit::LeaveCC,

[Rodolph Perfetta (ARM), 2016/06/29 14:45:08]

on arm32 INT32_MIN is a valid immediate so the following code is also one
instructions but removes the dependency on the input:

  __ mov(i.OutputRegister(), Operand(INT32_MIN), SBit::LeaveCC, vs);

[ahaas, 2016/06/29 15:07:33]

Done.

+             vs);
       DCHECK_EQ(LeaveCC, i.OutputSBit());
       break;
     }
     case kArmVcvtU32F32: {
       SwVfpRegister scratch = kScratchDoubleReg.low();
       __ vcvt_u32_f32(scratch, i.InputFloatRegister(0));
       __ vmov(i.OutputRegister(), scratch);
+      // Avoid UINT32_MAX as an overflow indicator and use 0 instead,
+      // because 0 allows easier out-of-bounds detection.
+      __ cmn(i.OutputRegister(), Operand(1));
+      __ add(i.OutputRegister(), i.OutputRegister(), Operand(1), SBit::LeaveCC,

[Rodolph Perfetta (ARM), 2016/06/29 14:45:08]

using adc would remove the dependency on the flags:

  __ adc(i.OutputRegister(), i.OutputRegister(), Operand::Zero());

[ahaas, 2016/06/29 14:49:42]

Is adc better than the mov instruction we use for the signed implementation,
e.g. __ mov(i.OutputRegister(), Operand(0), SBit::LeaveCC, cs)?

[ahaas, 2016/06/29 15:07:33]

Done.

+             cs);
       DCHECK_EQ(LeaveCC, i.OutputSBit());
       break;
     }
     case kArmVcvtS32F64: {
       SwVfpRegister scratch = kScratchDoubleReg.low();
       __ vcvt_s32_f64(scratch, i.InputDoubleRegister(0));
       __ vmov(i.OutputRegister(), scratch);
       DCHECK_EQ(LeaveCC, i.OutputSBit());
       break;
     }
@@ skipping 671 matching lines @@
       padding_size -= v8::internal::Assembler::kInstrSize;
     }
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8