1 files changed, 10 insertions, 9 deletions
diff --git a/llvm/lib/Target/X86/X86FixupBWInsts.cpp b/llvm/lib/Target/X86/X86FixupBWInsts.cpp
index 855ea683a8a..9a2f172aade 100644
--- a/llvm/lib/Target/X86/X86FixupBWInsts.cpp
+++ b/llvm/lib/Target/X86/X86FixupBWInsts.cpp
@@ -249,15 +249,16 @@ bool FixupBWInstPass::getSuperRegDestIfDead(MachineInstr *OrigMI,
 
     assert((MO.isDef() || MO.isUse()) && "Expected Def or Use only!");
 
-    for (MCSuperRegIterator Supers(OrigDestReg, TRI, true); Supers.isValid();
-         ++Supers) {
-      if (*Supers == MO.getReg()) {
-        if (MO.isDef())
-          IsDefined = true;
-        else
-          return false; // SuperReg Imp-used' -> live before the MI
-      }
-    }
+    if (MO.isDef() && TRI->isSuperRegisterEq(OrigDestReg, MO.getReg()))
+        IsDefined = true;
+
+    // If MO is a use of any part of the destination register but is not equal
+    // to OrigDestReg or one of its subregisters, we cannot use SuperDestReg.
+    // For example, if OrigDestReg is %al then an implicit use of %ah, %ax,
+    // %eax, or %rax will prevent us from using the %eax register.
+    if (MO.isUse() && !TRI->isSubRegisterEq(OrigDestReg, MO.getReg()) &&
+        TRI->regsOverlap(SuperDestReg, MO.getReg()))
+      return false;
   }
   // Reg is not Imp-def'ed -> it's live both before/after the instruction.
   if (!IsDefined)