+ added and implemented function TX86AsmOptimizer.Reg1WriteOverwritesReg2Entirely,

  which takes into account how 16-bit and 8-bit subregisters work on the x86
  (the new function is not used for now, but is probably going to replace
  SuperRegistersEqual in a few specific places (in RegLoadedWithNewValue),
  so that our register tracking becomes more accurate for 8-bit and 16-bit
  subregisters)

git-svn-id: trunk@35990 -

compiler/x86/aoptx86.pas

@@ -38,6 +38,9 @@ unit aoptx86;
       TX86AsmOptimizer = class(TAsmOptimizer)
         function RegLoadedWithNewValue(reg : tregister; hp : tai) : boolean; override;
       protected
+        { checks whether loading a new value in reg1 overwrites the entirety of reg2 }
+        function Reg1WriteOverwritesReg2Entirely(reg1, reg2: tregister): boolean;
+
         procedure PostPeepholeOptMov(const p : tai);
 
         function OptPass1AND(var p : tai) : boolean;
@@ -223,6 +226,43 @@ unit aoptx86;
 {$endif DEBUG_AOPTCPU}
 
 
+    function TX86AsmOptimizer.Reg1WriteOverwritesReg2Entirely(reg1, reg2: tregister): boolean;
+      begin
+        if not SuperRegistersEqual(reg1,reg2) then
+          exit(false);
+        if getregtype(reg1)<>R_INTREGISTER then
+          exit(true);  {because SuperRegisterEqual is true}
+        case getsubreg(reg1) of
+          { A write to R_SUBL doesn't change R_SUBH and if reg2 is R_SUBW or
+            higher, it preserves the high bits, so the new value depends on
+            reg2's previous value. In other words, it is equivalent to doing:
+
+            reg2 := (reg2 and $ffffff00) or byte(reg1); }
+          R_SUBL:
+            exit(getsubreg(reg2)=R_SUBL);
+          { A write to R_SUBH doesn't change R_SUBL and if reg2 is R_SUBW or
+            higher, it actually does a:
+
+            reg2 := (reg2 and $ffff00ff) or (reg1 and $ff00); }
+          R_SUBH:
+            exit(getsubreg(reg2)=R_SUBH);
+          { If reg2 is R_SUBD or larger, a write to R_SUBW preserves the high 16
+            bits of reg2:
+
+            reg2 := (reg2 and $ffff0000) or word(reg1); }
+          R_SUBW:
+            exit(getsubreg(reg2) in [R_SUBL,R_SUBH,R_SUBW]);
+          { a write to R_SUBD always overwrites every other subregister,
+            because it clears the high 32 bits of R_SUBQ on x86_64 }
+          R_SUBD,
+          R_SUBQ:
+            exit(true);
+          else
+            internalerror(2017042801);
+        end;
+      end;
+
+
     { allocates register reg between (and including) instructions p1 and p2
       the type of p1 and p2 must not be in SkipInstr
       note that this routine is both called from the peephole optimizer