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

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

Index: src/compiler/arm/code-generator-arm.cc
diff --git a/src/compiler/arm/code-generator-arm.cc b/src/compiler/arm/code-generator-arm.cc
index a9db0539b17d6a95e8ac21e72052d3baaacea7bf..7b4422a98fcd772c90691b930c111d92e8c953c1 100644
--- a/src/compiler/arm/code-generator-arm.cc
+++ b/src/compiler/arm/code-generator-arm.cc
@@ -1121,6 +1121,11 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       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;
     }
@@ -1128,6 +1133,11 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       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;
     }