ARM: Optimsisation of ECMA ToInt32.

src/arm/macro-assembler-arm.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2418 matching lines @@
 void MacroAssembler::SmiToDoubleVFPRegister(Register smi,
                                             DwVfpRegister value,
                                             Register scratch1,
                                             SwVfpRegister scratch2) {
   mov(scratch1, Operand(smi, ASR, kSmiTagSize));
   vmov(scratch2, scratch1);
   vcvt_f64_s32(value, scratch2);
 }
 
 
-// Tries to get a signed int32 out of a double precision floating point heap
-// number. Rounds towards 0. Branch to 'not_int32' if the double is out of the
-// 32bits signed integer range.
-void MacroAssembler::ConvertToInt32(Register source,
-                                    Register dest,
-                                    Register scratch,
-                                    Register scratch2,
-                                    DwVfpRegister double_scratch,
-                                    Label *not_int32) {
-  if (CpuFeatures::IsSupported(VFP2)) {
-    CpuFeatureScope scope(this, VFP2);
-    sub(scratch, source, Operand(kHeapObjectTag));
-    vldr(double_scratch, scratch, HeapNumber::kValueOffset);
-    vcvt_s32_f64(double_scratch.low(), double_scratch);
-    vmov(dest, double_scratch.low());
-    // Signed vcvt instruction will saturate to the minimum (0x80000000) or
-    // maximun (0x7fffffff) signed 32bits integer when the double is out of
-    // range. When substracting one, the minimum signed integer becomes the
-    // maximun signed integer.
-    sub(scratch, dest, Operand(1));
-    cmp(scratch, Operand(LONG_MAX - 1));
-    // If equal then dest was LONG_MAX, if greater dest was LONG_MIN.
-    b(ge, not_int32);
-  } else {
-    // This code is faster for doubles that are in the ranges -0x7fffffff to
-    // -0x40000000 or 0x40000000 to 0x7fffffff. This corresponds almost to
-    // the range of signed int32 values that are not Smis.  Jumps to the label
-    // 'not_int32' if the double isn't in the range -0x80000000.0 to
-    // 0x80000000.0 (excluding the endpoints).
-    Label right_exponent, done;
-    // Get exponent word.
-    ldr(scratch, FieldMemOperand(source, HeapNumber::kExponentOffset));
-    // Get exponent alone in scratch2.
-    Ubfx(scratch2,
-         scratch,
-         HeapNumber::kExponentShift,
-         HeapNumber::kExponentBits);
-    // Load dest with zero.  We use this either for the final shift or
-    // for the answer.
-    mov(dest, Operand::Zero());
-    // Check whether the exponent matches a 32 bit signed int that is not a Smi.
-    // A non-Smi integer is 1.xxx * 2^30 so the exponent is 30 (biased). This is
-    // the exponent that we are fastest at and also the highest exponent we can
-    // handle here.
-    const uint32_t non_smi_exponent = HeapNumber::kExponentBias + 30;
-    // The non_smi_exponent, 0x41d, is too big for ARM's immediate field so we
-    // split it up to avoid a constant pool entry.  You can't do that in general
-    // for cmp because of the overflow flag, but we know the exponent is in the
-    // range 0-2047 so there is no overflow.
-    int fudge_factor = 0x400;
-    sub(scratch2, scratch2, Operand(fudge_factor));
-    cmp(scratch2, Operand(non_smi_exponent - fudge_factor));
-    // If we have a match of the int32-but-not-Smi exponent then skip some
-    // logic.
-    b(eq, &right_exponent);
-    // If the exponent is higher than that then go to slow case.  This catches
-    // numbers that don't fit in a signed int32, infinities and NaNs.
-    b(gt, not_int32);
-
-    // We know the exponent is smaller than 30 (biased).  If it is less than
-    // 0 (biased) then the number is smaller in magnitude than 1.0 * 2^0, i.e.
-    // it rounds to zero.
-    const uint32_t zero_exponent = HeapNumber::kExponentBias + 0;
-    sub(scratch2, scratch2, Operand(zero_exponent - fudge_factor), SetCC);
-    // Dest already has a Smi zero.
-    b(lt, &done);
-
-    // We have an exponent between 0 and 30 in scratch2.  Subtract from 30 to
-    // get how much to shift down.
-    rsb(dest, scratch2, Operand(30));
-
-    bind(&right_exponent);
-    // Get the top bits of the mantissa.
-    and_(scratch2, scratch, Operand(HeapNumber::kMantissaMask));
-    // Put back the implicit 1.
-    orr(scratch2, scratch2, Operand(1 << HeapNumber::kExponentShift));
-    // Shift up the mantissa bits to take up the space the exponent used to
-    // take. We just orred in the implicit bit so that took care of one and
-    // we want to leave the sign bit 0 so we subtract 2 bits from the shift
-    // distance.
-    const int shift_distance = HeapNumber::kNonMantissaBitsInTopWord - 2;
-    mov(scratch2, Operand(scratch2, LSL, shift_distance));
-    // Put sign in zero flag.
-    tst(scratch, Operand(HeapNumber::kSignMask));
-    // Get the second half of the double. For some exponents we don't
-    // actually need this because the bits get shifted out again, but
-    // it's probably slower to test than just to do it.
-    ldr(scratch, FieldMemOperand(source, HeapNumber::kMantissaOffset));
-    // Shift down 22 bits to get the last 10 bits.
-    orr(scratch, scratch2, Operand(scratch, LSR, 32 - shift_distance));
-    // Move down according to the exponent.
-    mov(dest, Operand(scratch, LSR, dest));
-    // Fix sign if sign bit was set.
-    rsb(dest, dest, Operand::Zero(), LeaveCC, ne);
-    bind(&done);
-  }
-}
-
-
 void MacroAssembler::TestDoubleIsInt32(DwVfpRegister double_input,
                                        DwVfpRegister double_scratch) {
   ASSERT(!double_input.is(double_scratch));
   ASSERT(CpuFeatures::IsSupported(VFP2));
   CpuFeatureScope scope(this, VFP2);
 
   vcvt_s32_f64(double_scratch.low(), double_input);
   vcvt_f64_s32(double_scratch, double_scratch.low());
   VFPCompareAndSetFlags(double_input, double_scratch);
 }
@@ skipping 53 matching lines @@
   // floor(x) <=> round_to_zero(x) - 1.
   bind(&negative);
   sub(result, result, Operand(1), SetCC);
   // If result is still negative, go to done, result fetched.
   // Else, we had an overflow and we fall through exception.
   b(mi, done);
   bind(&exception);
 }
 
 
-void MacroAssembler::EmitOutOfInt32RangeTruncate(Register result,
-                                                 Register input_high,
-                                                 Register input_low,
-                                                 Register scratch) {
-  Label done, normal_exponent, restore_sign;
-
-  // Extract the biased exponent in result.
-  Ubfx(result,
-       input_high,
-       HeapNumber::kExponentShift,
-       HeapNumber::kExponentBits);
-
-  // Check for Infinity and NaNs, which should return 0.
-  cmp(result, Operand(HeapNumber::kExponentMask));
-  mov(result, Operand::Zero(), LeaveCC, eq);
-  b(eq, &done);
-
-  // Express exponent as delta to (number of mantissa bits + 31).
-  sub(result,
-      result,
-      Operand(HeapNumber::kExponentBias + HeapNumber::kMantissaBits + 31),
-      SetCC);
-
-  // If the delta is strictly positive, all bits would be shifted away,
-  // which means that we can return 0.
-  b(le, &normal_exponent);
-  mov(result, Operand::Zero());
-  b(&done);
-
-  bind(&normal_exponent);
-  const int kShiftBase = HeapNumber::kNonMantissaBitsInTopWord - 1;
-  // Calculate shift.
-  add(scratch, result, Operand(kShiftBase + HeapNumber::kMantissaBits), SetCC);
-
-  // Save the sign.
-  Register sign = result;
-  result = no_reg;
-  and_(sign, input_high, Operand(HeapNumber::kSignMask));
-
-  // Set the implicit 1 before the mantissa part in input_high.
-  orr(input_high,
-      input_high,
-      Operand(1 << HeapNumber::kMantissaBitsInTopWord));
-  // Shift the mantissa bits to the correct position.
-  // We don't need to clear non-mantissa bits as they will be shifted away.
-  // If they weren't, it would mean that the answer is in the 32bit range.
-  mov(input_high, Operand(input_high, LSL, scratch));
-
-  // Replace the shifted bits with bits from the lower mantissa word.
-  Label pos_shift, shift_done;
-  rsb(scratch, scratch, Operand(32), SetCC);
-  b(&pos_shift, ge);
-
-  // Negate scratch.
-  rsb(scratch, scratch, Operand::Zero());
-  mov(input_low, Operand(input_low, LSL, scratch));
-  b(&shift_done);
-
-  bind(&pos_shift);
-  mov(input_low, Operand(input_low, LSR, scratch));
-
-  bind(&shift_done);
-  orr(input_high, input_high, Operand(input_low));
-  // Restore sign if necessary.
-  cmp(sign, Operand::Zero());
-  result = sign;
-  sign = no_reg;
-  rsb(result, input_high, Operand::Zero(), LeaveCC, ne);
-  mov(result, input_high, LeaveCC, eq);
-  bind(&done);
+void MacroAssembler::ECMAConvertNumberToInt32(Register source,
+                                              Register result,
+                                              Register scratch,
+                                              Register input_high,
+                                              Register input_low,
+                                              DwVfpRegister double_scratch1,
+                                              DwVfpRegister double_scratch2) {
+  if (CpuFeatures::IsSupported(VFP2)) {
+    CpuFeatureScope scope(this, VFP2);
+    vldr(double_scratch1, FieldMemOperand(source, HeapNumber::kValueOffset));
+    ECMAToInt32VFP(result, double_scratch1, double_scratch2,
+                   scratch, input_high, input_low);
+  } else {
+    Ldrd(input_low, input_high,
+         FieldMemOperand(source, HeapNumber::kValueOffset));
+    ECMAToInt32NoVFP(result, scratch, input_high, input_low);
+  }
 }
 
 
-void MacroAssembler::EmitECMATruncate(Register result,
-                                      DwVfpRegister double_input,
-                                      DwVfpRegister double_scratch,
-                                      Register scratch,
-                                      Register input_high,
-                                      Register input_low) {
+void MacroAssembler::ECMAToInt32VFP(Register result,
+                                    DwVfpRegister double_input,
+                                    DwVfpRegister double_scratch,
+                                    Register scratch,
+                                    Register input_high,
+                                    Register input_low) {
   CpuFeatureScope scope(this, VFP2);
   ASSERT(!input_high.is(result));
   ASSERT(!input_low.is(result));
   ASSERT(!input_low.is(input_high));
   ASSERT(!scratch.is(result) &&
          !scratch.is(input_high) &&
          !scratch.is(input_low));
   ASSERT(!double_input.is(double_scratch));
 
-  Label done;
+  Label overflow, out_of_range, negate, done;
 
-  // Test if the value can be exactly represented as a signed integer.
-  TryDoubleToInt32Exact(result, double_input, double_scratch);
-  b(eq, &done);
+  vmov(input_low, input_high, double_input);
+  Ubfx(scratch, input_high,
+       HeapNumber::kExponentShift, HeapNumber::kExponentBits);
+  // Load scratch with exponent - 1. This is faster than loading
+  // with exponent because Bias + 1 = 1024 which is an *ARM* immediate value.
+  sub(scratch, scratch, Operand(HeapNumber::kExponentBias + 1));
+  // Compare exponent with 31 (compare exponent - 1 with 30).
+  cmp(scratch, Operand(30));
+  b(ge, &overflow);
+  // Exponent is less than 31 so vcvt will never saturate.
+  // So, just return the result.
+  vcvt_s32_f64(double_scratch.low(), double_input);
+  vmov(result, double_scratch.low());
+  b(&done);
 
-  // Check the exception flags. If they are not set, we are done.
-  // If they are set, it could be because of the conversion above, or because
-  // they were set before this code.
-  vmrs(scratch);
-  tst(scratch, Operand(kVFPInvalidOpExceptionBit));
-  b(eq, &done);
+  bind(&overflow);
+  // If exponent is greater than or equal to 84, the 32 less significant
+  // bits are 0s (2^84 = 1, 52 significant bits, 32 uncoded bits),
+  // the result is 0.
+  // This test also catch Nan and infinities which also return 0.
+  // Compare exponent with 84 (compare exponent - 1 with 83).
+  cmp(scratch, Operand(83));
+  b(ge, &out_of_range);
 
-  // Clear cumulative exception flags.
-  bic(scratch, scratch, Operand(kVFPInvalidOpExceptionBit));
-  vmsr(scratch);
-  // Try a conversion to a signed integer.
-  vcvt_s32_f64(double_scratch.low(), double_input);
-  // Retrieve the FPSCR.
-  vmrs(scratch);
-  // Check for invalid conversions (out of range and NaNs).
-  tst(scratch, Operand(kVFPInvalidOpExceptionBit));
-  // If we had no exceptions we are done.
-  b(eq, &done);
-
-  // Load the double value and perform a manual truncation.
-  vmov(input_low, input_high, double_input);
-  EmitOutOfInt32RangeTruncate(result,
-                              input_high,
-                              input_low,
-                              scratch);
-  bind(&done);
+  // If we reach this code, 31 <= exponent <= 83.
+  // So, we don't have to handle cases where 0 <= exponent <= 20 for
+  // which we would need to shift right the high part of the mantissa.
+  ECMAToInt32Tail(result, scratch, input_high, input_low,
+                  &out_of_range, &negate, &done);
 }
 
 
+void MacroAssembler::ECMAToInt32NoVFP(Register result,
+                                      Register scratch,
+                                      Register input_high,
+                                      Register input_low) {
+  ASSERT(!result.is(scratch));
+  ASSERT(!result.is(input_high));
+  ASSERT(!result.is(input_low));
+  ASSERT(!scratch.is(input_high));
+  ASSERT(!scratch.is(input_low));
+  ASSERT(!input_high.is(input_low));
+
+  Label both, out_of_range, negate, done;
+
+  Ubfx(scratch, input_high,
+       HeapNumber::kExponentShift, HeapNumber::kExponentBits);
+  // Load scratch with exponent - 1. This is faster than loading
+  // with exponent because Bias + 1 = 1024 which is an *ARM* immediate value.
+  sub(scratch, scratch, Operand(HeapNumber::kExponentBias + 1));
+  // If exponent is negative, 0 < input < 1, the result is 0.
+  // If exponent is greater than or equal to 84, the 32 less significant
+  // bits are 0s (2^84 = 1, 52 significant bits, 32 uncoded bits),
+  // the result is 0.
+  // This test also catch Nan and infinities which also return 0.
+  // Compare exponent with 84 (compare exponent - 1 with 83).
+  cmp(scratch, Operand(83));
+  // We do an unsigned comparison so negative numbers are treated as big
+  // positive number and the two tests above are done in one test.
+  b(hs, &out_of_range);
+
+  // Load scratch with 20 - exponent (load with 19 - (exponent - 1)).
+  rsb(scratch, scratch, Operand(19), SetCC);
+  b(mi, &both);
+
+  // 0 <= exponent <= 20, shift only input_high.
+  // Scratch contains: 20 - exponent.
+  Ubfx(result, input_high,
+       0, HeapNumber::kMantissaBitsInTopWord);
+  orr(result, result, Operand(1 << HeapNumber::kMantissaBitsInTopWord));
+  mov(result, Operand(result, LSR, scratch));
+  b(&negate);
+
+  bind(&both);
+  // Restore scratch to exponent - 1 to be consistent with ECMAToInt32VFP.
+  rsb(scratch, scratch, Operand(19));
+  ECMAToInt32Tail(result, scratch, input_high, input_low,
+                  &out_of_range, &negate, &done);
+}
+
+
+void MacroAssembler::ECMAToInt32Tail(Register result,
+                                     Register scratch,
+                                     Register input_high,
+                                     Register input_low,
+                                     Label* out_of_range,
+                                     Label* negate,
+                                     Label* done) {
+  Label only_low;
+
+  // On entry, scratch contains exponent - 1.
+  // Load scratch with 52 - exponent (load with 51 - (exponent - 1)).
+  rsb(scratch, scratch, Operand(51), SetCC);
+  b(ls, &only_low);
+  // 21 <= exponent <= 51, shift input_low and input_high
+  // to generate the result.
+  mov(input_low, Operand(input_low, LSR, scratch));
+  // Scratch contains: 52 - exponent.
+  // We needs: exponent - 20.
+  // So we use: 32 - scratch = 32 - 52 + exponent = exponent - 20.
+  rsb(scratch, scratch, Operand(32));
+  Ubfx(result, input_high,
+       0, HeapNumber::kMantissaBitsInTopWord);
+  orr(result, result, Operand(1 << HeapNumber::kMantissaBitsInTopWord));

[ulan, 2013/03/11 15:11:03]

Maybe restore the comment about implicit 1?
// Set the implicit 1 before the mantissa part in input_high.

[Rodolph Perfetta, 2013/03/12 11:24:45]

Done.

+  orr(result, input_low, Operand(result, LSL, scratch));
+  b(negate);
+
+  bind(out_of_range);
+  mov(result, Operand::Zero());
+  b(done);
+
+  bind(&only_low);
+  // 52 <= exponent <= 83, shift only input_low.
+  // On entry, scratch contains: 52 - exponent.
+  rsb(scratch, scratch, Operand::Zero());
+  mov(result, Operand(input_low, LSL, scratch));
+
+  bind(negate);
+  // If input was positive, input_high ASR 31 equals 0 and
+  // input_high LSR 31 equals zero.
+  // New result = (result eor 0) + 0 = result.
+  // If the input was negative, we have to negate the result.
+  // Input_high ASR 31 equals 0xffffffff and input_high LSR 31 equals 1.
+  // New result = (result eor 0xffffffff) + 1 = 0 - result.
+  eor(result, result, Operand(input_high, ASR, 31));

[ulan, 2013/03/11 15:11:03]

This is an awesome trick :)

+  add(result, result, Operand(input_high, LSR, 31));
+
+  bind(done);
+}
+
+
 void MacroAssembler::GetLeastBitsFromSmi(Register dst,
                                          Register src,
                                          int num_least_bits) {
   if (CpuFeatures::IsSupported(ARMv7) && !predictable_code_size()) {
     ubfx(dst, src, kSmiTagSize, num_least_bits);
   } else {
     mov(dst, Operand(src, ASR, kSmiTagSize));
     and_(dst, dst, Operand((1 << num_least_bits) - 1));
   }
 }
@@ skipping 1277 matching lines @@
 void CodePatcher::EmitCondition(Condition cond) {
   Instr instr = Assembler::instr_at(masm_.pc_);
   instr = (instr & ~kCondMask) | cond;
   masm_.emit(instr);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM