Optimize more easy multiplications by constants.

Rather than make these changes in the libraries (*10 being a common case),
let's do them once and for all in the JIT.

The 2^n-1 case could be better if we generated RSB instructions, but the
current "fake" RSB is still better than a full multiply.

Thumb doesn't support reg/reg/reg/shift instructions, so we can't optimize
the "population count <= 2" cases (such as *10) there.

Tested on sholes, passion, and passion-running-sapphire (and visually
inspected to check we weren't trying to generate Thumb2 instructions there).
Also tested with the self-verifier.

1 files changed, 11 insertions, 0 deletions

diff --git a/vm/compiler/codegen/arm/Thumb2/Gen.c b/vm/compiler/codegen/arm/Thumb2/Gen.c
index 1390d64c7..dbce45284 100644
--- a/vm/compiler/codegen/arm/Thumb2/Gen.c
+++ b/vm/compiler/codegen/arm/Thumb2/Gen.c
@@ -442,3 +442,14 @@ static bool genInlinedAbsLong(CompilationUnit *cUnit, MIR *mir)
     storeValueWide(cUnit, rlDest, rlResult);
     return false;
 }
+
+static void genMultiplyByTwoBitMultiplier(CompilationUnit *cUnit,
+        RegLocation rlSrc, RegLocation rlResult, int lit,
+        int firstBit, int secondBit)
+{
+    opRegRegRegShift(cUnit, kOpAdd, rlResult.lowReg, rlSrc.lowReg, rlSrc.lowReg,
+                     encodeShift(kArmLsl, secondBit - firstBit));
+    if (firstBit != 0) {
+        opRegRegImm(cUnit, kOpLsl, rlResult.lowReg, rlResult.lowReg, firstBit);
+    }
+}