LSRA-throughput: Iterate over the regMaskTP instead all registers

src/coreclr/jit/codegenarm64.cpp

@@ -708,16 +708,13 @@ void CodeGen::genBuildRegPairsStack(regMaskTP regsMask, ArrayStack<RegPair>* reg
 
     while (regsMask != RBM_NONE)
     {
-        regMaskTP reg1Mask = genFindLowestBit(regsMask);
-        regNumber reg1     = genRegNumFromMask(reg1Mask);
-        regsMask &= ~reg1Mask;
+        regNumber reg1 = genFirstRegNumFromMaskAndToggle(regsMask);
         regsCount -= 1;
 
         bool isPairSave = false;
         if (regsCount > 0)
         {
-            regMaskTP reg2Mask = genFindLowestBit(regsMask);
-            regNumber reg2     = genRegNumFromMask(reg2Mask);
+            regNumber reg2 = genFirstRegNumFromMask(regsMask);
             if (reg2 == REG_NEXT(reg1))
             {
                 // The JIT doesn't allow saving pair (R28,FP), even though the
@@ -733,7 +730,7 @@ void CodeGen::genBuildRegPairsStack(regMaskTP regsMask, ArrayStack<RegPair>* reg
                     {
                         isPairSave = true;
 
-                        regsMask &= ~reg2Mask;
+                        regsMask &= ~genRegMask(reg2);
                         regsCount -= 1;
 
                         regStack->Push(RegPair(reg1, reg2));

src/coreclr/jit/compiler.hpp

@@ -706,6 +706,50 @@ inline regNumber genRegNumFromMask(regMaskTP mask)
     return regNum;
 }
 
+//------------------------------------------------------------------------------
+// genFirstRegNumFromMaskAndToggle : Maps first bit set in the register mask to a
+//          register number and also toggle the bit in the `mask`.
+// Arguments:
+//    mask               - the register mask
+//
+// Return Value:
+//    The number of the first register contained in the mask and updates the `mask` to toggle
+//    the bit.
+//
+
+inline regNumber genFirstRegNumFromMaskAndToggle(regMaskTP& mask)
+{
+    assert(mask != 0); // Must have one bit set, so can't have a mask of zero
+
+    /* Convert the mask to a register number */
+
+    regNumber regNum = (regNumber)BitOperations::BitScanForward(mask);
+    mask ^= genRegMask(regNum);
+
+    return regNum;
+}
+
+//------------------------------------------------------------------------------
+// genFirstRegNumFromMask : Maps first bit set in the register mask to a register number.
+//
+// Arguments:
+//    mask               - the register mask
+//
+// Return Value:
+//    The number of the first register contained in the mask.
+//
+
+inline regNumber genFirstRegNumFromMask(regMaskTP mask)
+{
+    assert(mask != 0); // Must have one bit set, so can't have a mask of zero
+
+    /* Convert the mask to a register number */
+
+    regNumber regNum = (regNumber)BitOperations::BitScanForward(mask);
+
+    return regNum;
+}
+
 /*****************************************************************************
  *
  *  Return the size in bytes of the given type.

src/coreclr/jit/gentree.cpp

@@ -25802,9 +25802,9 @@ regNumber GenTree::ExtractTempReg(regMaskTP mask /* = (regMaskTP)-1 */)
 {
     regMaskTP availableSet = gtRsvdRegs & mask;
     assert(genCountBits(availableSet) >= 1);
-    regMaskTP tempRegMask = genFindLowestBit(availableSet);
-    gtRsvdRegs &= ~tempRegMask;
-    return genRegNumFromMask(tempRegMask);
+    regNumber tempReg = genFirstRegNumFromMask(availableSet);
+    gtRsvdRegs ^= genRegMask(tempReg);
+    return tempReg;
 }
 
 //------------------------------------------------------------------------