MachineCSE: Update the heuristics for isProfitableToCSE.

If the result of a common subexpression is used at all uses of the candidate
expression, CSE should not increase the live range of the common subexpression.

rdar://11393714 and rdar://11819721


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161396 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineCSE.cpp

@@ -324,6 +324,29 @@ bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
                                    MachineInstr *CSMI, MachineInstr *MI) {
   // FIXME: Heuristics that works around the lack the live range splitting.
 
+  // If CSReg is used at all uses of Reg, CSE should not increase register
+  // pressure of CSReg.
+  bool MayIncreasePressure = true;
+  if (TargetRegisterInfo::isVirtualRegister(CSReg) &&
+      TargetRegisterInfo::isVirtualRegister(Reg)) {
+    MayIncreasePressure = false;
+    SmallPtrSet<MachineInstr*, 8> CSUses;
+    for (MachineRegisterInfo::use_nodbg_iterator I =MRI->use_nodbg_begin(CSReg),
+         E = MRI->use_nodbg_end(); I != E; ++I) {
+      MachineInstr *Use = &*I;
+      CSUses.insert(Use);
+    }
+    for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
+         E = MRI->use_nodbg_end(); I != E; ++I) {
+      MachineInstr *Use = &*I;
+      if (!CSUses.count(Use)) {
+        MayIncreasePressure = true;
+        break;
+      }
+    }
+  }
+  if (!MayIncreasePressure) return true;
+
   // Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
   // an immediate predecessor. We don't want to increase register pressure and
   // end up causing other computation to be spilled.

test/CodeGen/X86/block-placement.ll

@@ -634,7 +634,7 @@ define void @test_unnatural_cfg_backwards_inner_loop() {
 ;
 ; CHECK: test_unnatural_cfg_backwards_inner_loop
 ; CHECK: %entry
-; CHECK: %body
+; CHECK: [[BODY:# BB#[0-9]+]]:
 ; CHECK: %loop2b
 ; CHECK: %loop1
 ; CHECK: %loop2a

test/CodeGen/X86/machine-cse.ll

@@ -99,3 +99,38 @@ return:                                           ; preds = %if.end, %entry
   %retval.0 = phi i32 [ 1, %entry ], [ %., %if.end ]
   ret i32 %retval.0
 }
+
+; rdar://11393714
+define i8* @bsd_memchr(i8* %s, i32 %a, i32 %c, i64 %n) nounwind ssp {
+; CHECK: %entry
+; CHECK: xorl
+; CHECK: %preheader
+; CHECK: %do.body
+; CHECK-NOT: xorl
+; CHECK: %do.cond
+; CHECK-NOT: xorl
+; CHECK: %return
+entry:
+  %cmp = icmp eq i64 %n, 0
+  br i1 %cmp, label %return, label %preheader
+
+preheader:
+  %conv2 = and i32 %c, 255
+  br label %do.body
+
+do.body:
+  %n.addr.0 = phi i64 [ %dec, %do.cond ], [ %n, %preheader ]
+  %p.0 = phi i8* [ %incdec.ptr, %do.cond ], [ %s, %preheader ]
+  %cmp3 = icmp eq i32 %a, %conv2
+  br i1 %cmp3, label %return, label %do.cond
+
+do.cond:
+  %incdec.ptr = getelementptr inbounds i8* %p.0, i64 1
+  %dec = add i64 %n.addr.0, -1
+  %cmp6 = icmp eq i64 %dec, 0
+  br i1 %cmp6, label %return, label %do.body
+
+return:
+  %retval.0 = phi i8* [ null, %entry ], [ null, %do.cond ], [ %p.0, %do.body ]
+  ret i8* %retval.0
+}