Reviewers: ulan, jochen,
Description:
ARM64: Optimize MathRoundI
Optimize code generated for MathRoundI based on technique used in
MathRoundD.
BUG=
Please review this at https://codereview.chromium.org/276283003/
SVN Base: https://v8.googlecode.com/svn/branches/bleeding_edge
Affected files (+30, -31 lines):
M src/arm64/lithium-codegen-arm64.cc
Index: src/arm64/lithium-codegen-arm64.cc
diff --git a/src/arm64/lithium-codegen-arm64.cc
b/src/arm64/lithium-codegen-arm64.cc
index
ef20c67f80c4fc508f259720a88d6a06cd6d35d6..c1e971d5b567f981ec5efc5f4f59e36967b30f1d
100644
--- a/src/arm64/lithium-codegen-arm64.cc
+++ b/src/arm64/lithium-codegen-arm64.cc
@@ -4111,9 +4111,9 @@ void LCodeGen::DoMathRoundD(LMathRoundD* instr) {
void LCodeGen::DoMathRoundI(LMathRoundI* instr) {
DoubleRegister input = ToDoubleRegister(instr->value());
- DoubleRegister temp1 = ToDoubleRegister(instr->temp1());
+ DoubleRegister temp = ToDoubleRegister(instr->temp1());
+ DoubleRegister dot_five = double_scratch();
Register result = ToRegister(instr->result());
- Label try_rounding;
Label done;
// Math.round() rounds to the nearest integer, with ties going towards
@@ -4124,42 +4124,41 @@ void LCodeGen::DoMathRoundI(LMathRoundI* instr) {
// that -0.0 rounds to itself, and values -0.5 <= input < 0 also
produce a
// result of -0.0.
- DoubleRegister dot_five = double_scratch();
+ // Add 0.5 and round towards -infinity.
__ Fmov(dot_five, 0.5);
- __ Fabs(temp1, input);
- __ Fcmp(temp1, dot_five);
- // If input is in [-0.5, -0], the result is -0.
- // If input is in [+0, +0.5[, the result is +0.
- // If the input is +0.5, the result is 1.
- __ B(hi, &try_rounding); // hi so NaN will also branch.
+ __ Fadd(temp, input, dot_five);
+ __ Fcvtms(result, temp);
+
+ // The result is correct if:
+ // result is not 0, as the input could be NaN or [-0.5, -0.0].
+ // result is not 1, as 0.499...94 will wrongly map to 1.
+ // result fits in 32 bits.
+ __ Cmp(result, Operand(result.W(), SXTW));
+ __ Ccmp(result, 1, ZFlag, eq);
+ __ B(hi, &done);
+
+ // At this point, we have to handle possible inputs of NaN or numbers in
the
+ // range [-0.5, 1.5[, or numbers larger than 32 bits.
+
+ // Deoptimize if the result > 1, as it must be larger than 32 bits.
+ __ Cmp(result, 1);
+ DeoptimizeIf(hi, instr->environment());
+ // Deoptimize for negative inputs, which at this point are only numbers
in
+ // the range [-0.5, -0.0]
if (instr->hydrogen()->CheckFlag(HValue::kBailoutOnMinusZero)) {
__ Fmov(result, input);
- DeoptimizeIfNegative(result, instr->environment()); // [-0.5, -0.0].
+ DeoptimizeIfNegative(result, instr->environment());
}
- __ Fcmp(input, dot_five);
- __ Mov(result, 1); // +0.5.
- // Remaining cases: [+0, +0.5[ or [-0.5, +0.5[, depending on
- // flag kBailoutOnMinusZero, will return 0 (xzr).
- __ Csel(result, result, xzr, eq);
- __ B(&done);
- __ Bind(&try_rounding);
- // Since we're providing a 32-bit result, we can implement
ties-to-infinity by
- // adding 0.5 to the input, then taking the floor of the result. This
does not
- // work for very large positive doubles because adding 0.5 would cause an
- // intermediate rounding stage, so a different approach is necessary
when a
- // double result is needed.
- __ Fadd(temp1, input, dot_five);
- __ Fcvtms(result, temp1);
-
- // Deopt if
- // * the input was NaN
- // * the result is not representable using a 32-bit integer.
- __ Fcmp(input, 0.0);
- __ Ccmp(result, Operand(result.W(), SXTW), NoFlag, vc);
- DeoptimizeIf(ne, instr->environment());
+ // Deoptimize if the input was NaN.
+ __ Fcmp(input, dot_five);
+ DeoptimizeIf(vs, instr->environment());
+ // Now, the only unhandled inputs are in the range [0.0, 1.5[ (or [-0.5,
1.5[
+ // if we didn't generate a -0.0 bailout). If input >= 0.5 then return 1,
+ // else 0; we avoid dealing with 0.499...94 directly.
+ __ Cset(result, ge);
__ Bind(&done);
}
--
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