x86/x64 port of Math.floor(x/y) to use integer division for specific divisor

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
===================================================================
--- src/ia32/lithium-codegen-ia32.cc	(revision 11554)
+++ src/ia32/lithium-codegen-ia32.cc	(working copy)
@@ -963,6 +963,106 @@
 }
 
 
+void LCodeGen::DoMathFloorOfDiv(LMathFloorOfDiv* instr) {
+  ASSERT(instr->InputAt(1)->IsConstantOperand());
+
+  Register dividend = ToRegister(instr->InputAt(0));
+  int32_t divisor = ToInteger32(LConstantOperand::cast(instr->InputAt(1)));
+  Register result = ToRegister(instr->result());
+
+  switch (divisor) {
+  case 0:
+    DeoptimizeIf(no_condition, instr->environment());
+    return;
+
+  case 1:
+    __ Move(result, dividend);
+    return;
+
+  case -1:
+    __ Move(result, dividend);
+    __ neg(result);
+    // FIXME: set & check HValue::kBailoutOnMinusZero

[Yang, 2012/06/19 15:41:38]

Is this going to be fixed?

+    DeoptimizeIf(zero, instr->environment());
+    if (instr->hydrogen()->CheckFlag(HValue::kCanOverflow)) {
+      DeoptimizeIf(overflow, instr->environment());
+    }
+    return;
+  }
+
+  uint32_t divisor_abs = abs(divisor);
+  if (IsPowerOf2(divisor_abs)) {
+    int32_t power = WhichPowerOf2(divisor_abs);
+    if (divisor < 0) {
+      // input[dividend] is clobbered
+      // tedious because neg(dividend) might overflow

[Yang, 2012/06/19 15:41:38]

Comments should start with capital letter and end with a period.

Wouldn't neg only overflow for -2^31? Can't we simply deoptimize for this case
and do a simple neg for the rest?

+      __ mov(result, dividend);
+      __ sar(dividend, 31);
+      __ neg(result);
+      // FIXME: set & check HValue::kBailoutOnMinusZero
+      DeoptimizeIf(zero, instr->environment());
+      __ shl(dividend, 32 - power);
+      __ sar(result, power);
+      __ not_(dividend);
+      // clear result.sign if dividend.sign
+      __ and_(result, dividend);
+    } else {
+      __ Move(result, dividend);
+      __ sar(result, power);
+    }
+  } else {
+    ASSERT(ToRegister(instr->InputAt(0)).is(eax));
+    ASSERT(ToRegister(instr->result()).is(edx));
+    Register scratch = ToRegister(instr->TempAt(0));
+
+    // b: 2^b < divisor_abs < 2^(b+1)
+    unsigned b = 31 - CompilerIntrinsics::CountLeadingZeros(divisor_abs);
+    unsigned shift = 32 + b;  // precision +1bit (effectively)
+    double multiplier_f = static_cast<double>((uint64_t)1 << shift)

[Yang, 2012/06/19 15:41:38]

Use static_cast please.

+                            / divisor_abs;
+    int64_t multiplier;
+    if (multiplier_f - floor(multiplier_f) < 0.5) {
+        multiplier = floor(multiplier_f);
+    } else {
+        multiplier = floor(multiplier_f) + 1;
+    }
+    // multiplier is a uint32
+    ASSERT(multiplier > 0 && multiplier < ((int64_t)1<<32));

[Yang, 2012/06/19 15:41:38]

Use static_cast please.

+    __ mov(scratch, dividend);
+    // FIXME: set & check HValue::kBailoutOnMinusZero

[Yang, 2012/06/19 15:41:38]

Same here.

+    if (divisor < 0) {
+      __ test(dividend, dividend);
+      DeoptimizeIf(zero, instr->environment());
+    }
+    __ mov(edx, multiplier);
+    __ imul(edx);
+    if ((int32_t)multiplier < 0) {

[Yang, 2012/06/19 15:41:38]

Use static_cast please.

+      __ add(edx, scratch);
+    }
+    Register reg_lo = eax;
+    Register reg_byte_scratch = scratch;
+    if (!reg_byte_scratch.is_byte_register()) {
+        __ xchg(reg_lo, reg_byte_scratch);
+        reg_lo = scratch;
+        reg_byte_scratch = eax;
+    }
+    if (divisor < 0) {
+      __ xor_(reg_byte_scratch, reg_byte_scratch);
+      __ cmp(reg_lo, 0x40000000);
+      __ setcc(above, reg_byte_scratch);
+      __ neg(edx);
+      __ sub(edx, reg_byte_scratch);
+    } else {
+      __ xor_(reg_byte_scratch, reg_byte_scratch);
+      __ cmp(reg_lo, 0xC0000000);
+      __ setcc(above_equal, reg_byte_scratch);
+      __ add(edx, reg_byte_scratch);
+    }
+    __ sar(edx, shift - 32);
+  }
+}
+
+
 void LCodeGen::DoMulI(LMulI* instr) {
   Register left = ToRegister(instr->InputAt(0));
   LOperand* right = instr->InputAt(1);