From d0fe2952f3589bc15edb232fd47b937d761d2476 Mon Sep 17 00:00:00 2001
From: "ulan@chromium.org"
 <ulan@chromium.org@ce2b1a6d-e550-0410-aec6-3dcde31c8c00>
Date: Tue, 25 Mar 2014 16:09:29 +0000
Subject: [PATCH] Revert r20246 "ARM64: Add overflow checking support for
 multiplications by constant powers of 2."

Reason: mjsunit/mul-exhaustive-part4 fails in no snaphot debug.

TBR=alexandre.rames@arm.com

Review URL: https://codereview.chromium.org/211393002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20254 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
---
 src/arm64/lithium-arm64.cc         | 23 +++++++------------
 src/arm64/lithium-codegen-arm64.cc | 47 ++++++++++++--------------------------
 2 files changed, 23 insertions(+), 47 deletions(-)

diff --git a/src/arm64/lithium-arm64.cc b/src/arm64/lithium-arm64.cc
index 3d3ae97..c6f547f 100644
--- a/src/arm64/lithium-arm64.cc
+++ b/src/arm64/lithium-arm64.cc
@@ -1891,36 +1891,29 @@ LInstruction* LChunkBuilder::DoMul(HMul* instr) {
     HValue* least_const = instr->BetterLeftOperand();
     HValue* most_const = instr->BetterRightOperand();
 
-    LOperand* left;
+    LOperand* left = UseRegisterAtStart(least_const);
 
     // LMulConstI can handle a subset of constants:
     //  With support for overflow detection:
     //    -1, 0, 1, 2
-    //    2^n, -(2^n)
     //  Without support for overflow detection:
+    //    2^n, -(2^n)
     //    2^n + 1, -(2^n - 1)
     if (most_const->IsConstant()) {
       int32_t constant = HConstant::cast(most_const)->Integer32Value();
-      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))))) {
+      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)))) {
         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;
diff --git a/src/arm64/lithium-codegen-arm64.cc b/src/arm64/lithium-codegen-arm64.cc
index 5b0b018..d61151e 100644
--- a/src/arm64/lithium-codegen-arm64.cc
+++ b/src/arm64/lithium-codegen-arm64.cc
@@ -4253,7 +4253,6 @@ 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 =
@@ -4297,40 +4296,20 @@ 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:
-      // Multiplication by constant powers of two (and some related values)
-      // can be done efficiently with shifted operands.
-      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, right_log2);
-        } else {
-          // result = -left << log2(-right)
-          __ 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);
 
+      // Multiplication by constant powers of two (and some related values)
+      // can be done efficiently with shifted operands.
       if (right >= 0) {
-        if (IsPowerOf2(right - 1)) {
+        if (IsPowerOf2(right)) {
+          // result = left << log2(right)
+          __ Lsl(result, left, WhichPowerOf2(right));
+        } else if (IsPowerOf2(right - 1)) {
           // result = left + left << log2(right - 1)
           __ Add(result, left, Operand(left, LSL, WhichPowerOf2(right - 1)));
         } else if (IsPowerOf2(right + 1)) {
@@ -4341,7 +4320,10 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
           UNREACHABLE();
         }
       } else {
-        if (IsPowerOf2(-right + 1)) {
+        if (IsPowerOf2(-right)) {
+          // result = -left << log2(-right)
+          __ Neg(result, Operand(left, LSL, WhichPowerOf2(-right)));
+        } else if (IsPowerOf2(-right + 1)) {
           // result = left - left << log2(-right + 1)
           __ Sub(result, left, Operand(left, LSL, WhichPowerOf2(-right + 1)));
         } else if (IsPowerOf2(-right - 1)) {
@@ -4352,6 +4334,7 @@ void LCodeGen::DoMulConstIS(LMulConstIS* instr) {
           UNREACHABLE();
         }
       }
+      break;
   }
 }
 
-- 
2.7.4