Reviewers: ulan, jochen,
Message:
Created a new issue for the re-land.
Description:
Reland "ARM64: Add overflow checking support for multiplications by constant
powers of 2."
This includes fixes for the overflow case spotted in
mjsunit/mul-exhaustive-part4.
Please review this at https://codereview.chromium.org/212573002/
SVN Base: https://v8.googlecode.com/svn/branches/bleeding_edge
Affected files (+52, -23 lines):
M src/arm64/lithium-arm64.cc
M src/arm64/lithium-codegen-arm64.cc
Index: src/arm64/lithium-arm64.cc
diff --git a/src/arm64/lithium-arm64.cc b/src/arm64/lithium-arm64.cc
index
c6f547f6284db0dcccc091f52f21b02f3f9c8688..3d3ae97b0caab0f3c79e9d030a0737502b137a18
100644
--- a/src/arm64/lithium-arm64.cc
+++ b/src/arm64/lithium-arm64.cc
@@ -1891,29 +1891,36 @@ LInstruction* LChunkBuilder::DoMul(HMul* instr) {
HValue* least_const = instr->BetterLeftOperand();
HValue* most_const = instr->BetterRightOperand();
- LOperand* left = UseRegisterAtStart(least_const);
+ LOperand* left;
// LMulConstI can handle a subset of constants:
// With support for overflow detection:
// -1, 0, 1, 2
- // Without support for overflow detection:
// 2^n, -(2^n)
+ // Without support for overflow detection:
// 2^n + 1, -(2^n - 1)
if (most_const->IsConstant()) {
int32_t constant = HConstant::cast(most_const)->Integer32Value();
- int32_t constant_abs = (constant >= 0) ? constant : -constant;
-
- if (((constant >= -1) && (constant <= 2)) ||
- (!can_overflow && (IsPowerOf2(constant_abs) ||
- IsPowerOf2(constant_abs + 1) ||
- IsPowerOf2(constant_abs - 1)))) {
+ bool small_constant = (constant >= -1) && (constant <= 2);
+ bool end_range_constant = (constant <= -kMaxInt) || (constant ==
kMaxInt);
+ int32_t constant_abs = Abs(constant);
+
+ if (!end_range_constant &&
+ (small_constant ||
+ (IsPowerOf2(constant_abs)) ||
+ (!can_overflow && (IsPowerOf2(constant_abs + 1) ||
+ IsPowerOf2(constant_abs - 1))))) {
LConstantOperand* right = UseConstant(most_const);
+ bool need_register = IsPowerOf2(constant_abs) && !small_constant;
+ left = need_register ? UseRegister(least_const)
+ : UseRegisterAtStart(least_const);
LMulConstIS* mul = new(zone()) LMulConstIS(left, right);
if (needs_environment) AssignEnvironment(mul);
return DefineAsRegister(mul);
}
}
+ left = UseRegisterAtStart(least_const);
// LMulI/S can handle all cases, but it requires that a register is
// allocated for the second operand.
LInstruction* result;
Index: src/arm64/lithium-codegen-arm64.cc
diff --git a/src/arm64/lithium-codegen-arm64.cc
b/src/arm64/lithium-codegen-arm64.cc
index
d61151eec3824aac9b725a31aef1c8cc61548d93..b1dca1f13d3b95d7ce5cd0ab43c8cd51d0995dae
100644
--- a/src/arm64/lithium-codegen-arm64.cc
+++ b/src/arm64/lithium-codegen-arm64.cc
@@ -4253,6 +4253,7 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
Register left =
is_smi ? ToRegister(instr->left()) : ToRegister32(instr->left()) ;
int32_t right = ToInteger32(instr->right());
+ ASSERT((right > -kMaxInt) || (right < kMaxInt));
bool can_overflow = instr->hydrogen()->CheckFlag(HValue::kCanOverflow);
bool bailout_on_minus_zero =
@@ -4296,20 +4297,45 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
}
break;
- // All other cases cannot detect overflow, because it would probably
be no
- // faster than using the smull method in LMulI.
- // TODO(jbramley): Investigate this, and add overflow support if it
would
- // be useful.
default:
- ASSERT(!can_overflow);
-
// Multiplication by constant powers of two (and some related values)
// can be done efficiently with shifted operands.
- if (right >= 0) {
- if (IsPowerOf2(right)) {
+ int32_t right_abs = Abs(right);
+
+ if (IsPowerOf2(right_abs)) {
+ int right_log2 = WhichPowerOf2(right_abs);
+
+ if (can_overflow) {
+ Register scratch = result;
+ ASSERT(!AreAliased(scratch, left));
+ __ Cls(scratch, left);
+ __ Cmp(scratch, right_log2);
+ DeoptimizeIf(lt, instr->environment());
+ }
+
+ if (right >= 0) {
// result = left << log2(right)
- __ Lsl(result, left, WhichPowerOf2(right));
- } else if (IsPowerOf2(right - 1)) {
+ __ Lsl(result, left, right_log2);
+ } else {
+ // result = -left << log2(-right)
+ if (can_overflow) {
+ __ Negs(result, Operand(left, LSL, right_log2));
+ DeoptimizeIf(vs, instr->environment());
+ } else {
+ __ Neg(result, Operand(left, LSL, right_log2));
+ }
+ }
+ return;
+ }
+
+
+ // For the following cases, we could perform a conservative overflow
check
+ // with CLS as above. However the few cycles saved are likely not
worth
+ // the risk of deoptimizing more often than required.
+ ASSERT(!can_overflow);
+
+ if (right >= 0) {
+ if (IsPowerOf2(right - 1)) {
// result = left + left << log2(right - 1)
__ Add(result, left, Operand(left, LSL, WhichPowerOf2(right -
1)));
} else if (IsPowerOf2(right + 1)) {
@@ -4320,10 +4346,7 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
UNREACHABLE();
}
} else {
- if (IsPowerOf2(-right)) {
- // result = -left << log2(-right)
- __ Neg(result, Operand(left, LSL, WhichPowerOf2(-right)));
- } else if (IsPowerOf2(-right + 1)) {
+ if (IsPowerOf2(-right + 1)) {
// result = left - left << log2(-right + 1)
__ Sub(result, left, Operand(left, LSL, WhichPowerOf2(-right +
1)));
} else if (IsPowerOf2(-right - 1)) {
@@ -4334,7 +4357,6 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
UNREACHABLE();
}
}
- break;
}
}
--
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