Optimizing/ARM: Fix CmpConstant().

CMN updates flags based on addition of its operands.
Do not confuse the "N" suffix with bitwise inversion
performed by MVN.

Also add more special cases analogous to AddConstant()
and use CmpConstant() more in code generator.

Change-Id: I0d4571770a3f0fdf162e97d4bde56814098e7246

1 files changed, 21 insertions, 11 deletions

diff --git a/compiler/utils/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index f341030c15..52023a67ee 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -3428,10 +3428,10 @@ void Thumb2Assembler::AddConstant(Register rd, Register rn, int32_t value,
     CHECK(rn != IP);
     // If rd != rn, use rd as temp. This alows 16-bit ADD/SUB in more situations than using IP.
     Register temp = (rd != rn) ? rd : IP;
-    if (ShifterOperandCanHold(temp, kNoRegister, MVN, ~value, set_cc, &shifter_op)) {
+    if (ShifterOperandCanHold(temp, kNoRegister, MVN, ~value, kCcKeep, &shifter_op)) {
       mvn(temp, shifter_op, cond, kCcKeep);
       add(rd, rn, ShifterOperand(temp), cond, set_cc);
-    } else if (ShifterOperandCanHold(temp, kNoRegister, MVN, ~(-value), set_cc, &shifter_op)) {
+    } else if (ShifterOperandCanHold(temp, kNoRegister, MVN, ~(-value), kCcKeep, &shifter_op)) {
       mvn(temp, shifter_op, cond, kCcKeep);
       sub(rd, rn, ShifterOperand(temp), cond, set_cc);
     } else if (High16Bits(-value) == 0) {
@@ -3449,22 +3449,32 @@ void Thumb2Assembler::AddConstant(Register rd, Register rn, int32_t value,
 }
 
 void Thumb2Assembler::CmpConstant(Register rn, int32_t value, Condition cond) {
-  // We prefer to select the shorter code sequence rather than selecting add for
-  // positive values and sub for negatives ones, which would slightly improve
-  // the readability of generated code for some constants.
+  // We prefer to select the shorter code sequence rather than using plain cmp and cmn
+  // which would slightly improve the readability of generated code for some constants.
   ShifterOperand shifter_op;
   if (ShifterOperandCanHold(kNoRegister, rn, CMP, value, kCcSet, &shifter_op)) {
     cmp(rn, shifter_op, cond);
-  } else if (ShifterOperandCanHold(kNoRegister, rn, CMN, ~value, kCcSet, &shifter_op)) {
+  } else if (ShifterOperandCanHold(kNoRegister, rn, CMN, -value, kCcSet, &shifter_op)) {
     cmn(rn, shifter_op, cond);
   } else {
     CHECK(rn != IP);
-    movw(IP, Low16Bits(value), cond);
-    uint16_t value_high = High16Bits(value);
-    if (value_high != 0) {
-      movt(IP, value_high, cond);
+    if (ShifterOperandCanHold(IP, kNoRegister, MVN, ~value, kCcKeep, &shifter_op)) {
+      mvn(IP, shifter_op, cond, kCcKeep);
+      cmp(rn, ShifterOperand(IP), cond);
+    } else if (ShifterOperandCanHold(IP, kNoRegister, MVN, ~(-value), kCcKeep, &shifter_op)) {
+      mvn(IP, shifter_op, cond, kCcKeep);
+      cmn(rn, ShifterOperand(IP), cond);
+    } else if (High16Bits(-value) == 0) {
+      movw(IP, Low16Bits(-value), cond);
+      cmn(rn, ShifterOperand(IP), cond);
+    } else {
+      movw(IP, Low16Bits(value), cond);
+      uint16_t value_high = High16Bits(value);
+      if (value_high != 0) {
+        movt(IP, value_high, cond);
+      }
+      cmp(rn, ShifterOperand(IP), cond);
     }
-    cmp(rn, ShifterOperand(IP), cond);
   }
 }