LoopUnswitch - Simplify based on known not to a be constant.

Summary: In case we do not know what the condition is in an unswitched loop, but we know its definitely NOT a known constant. We can perform simplifcations based on this information.

Reviewers: sanjoy, hfinkel, chenli, efriedma

Reviewed By: efriedma

Subscribers: david2050, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D28968

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296041 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopUnswitch.cpp

@@ -47,6 +47,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/Support/CommandLine.h"
@@ -235,6 +236,11 @@ namespace {
                                         TerminatorInst *TI);
 
     void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
+
+    /// Given that the Invariant is not equal to Val. Simplify instructions
+    /// in the loop.
+    Value *SimplifyInstructionWithNotEqual(Instruction *Inst, Value *Invariant,
+                                           Constant *Val);
   };
 }
 
@@ -1218,10 +1224,21 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
     if (!UI || !L->contains(UI))
       continue;
 
-    Worklist.push_back(UI);
+    // At this point, we know LIC is definitely not Val. Try to use some simple
+    // logic to simplify the user w.r.t. to the context.
+    if (Value *Replacement = SimplifyInstructionWithNotEqual(UI, LIC, Val)) {
+      if (LI->replacementPreservesLCSSAForm(UI, Replacement)) {
+        // This in-loop instruction has been simplified w.r.t. its context,
+        // i.e. LIC != Val, make sure we propagate its replacement value to
+        // all its users.
+        ReplaceUsesOfWith(UI, Replacement, Worklist, L, LPM);
+        continue;
+      }
+    }
 
-    // TODO: We could do other simplifications, for example, turning
-    // 'icmp eq LIC, Val' -> false.
+    // Unable to simplify with non-valueness, push it into the worklist so that
+    // SimplifyCode can attempt to simplify it.
+    Worklist.push_back(UI);
 
     // If we know that LIC is not Val, use this info to simplify code.
     SwitchInst *SI = dyn_cast<SwitchInst>(UI);
@@ -1362,3 +1379,27 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
     }
   }
 }
+
+/// Simple simplifications we can do given the information that Cond is
+/// definitely not equal to Val.
+Value *LoopUnswitch::SimplifyInstructionWithNotEqual(Instruction *Inst,
+                                                     Value *Invariant,
+                                                     Constant *Val) {
+  // icmp eq cond, val -> false
+  ICmpInst *CI = dyn_cast<ICmpInst>(Inst);
+  if (CI && CI->isEquality()) {
+    Value *Op0 = CI->getOperand(0);
+    Value *Op1 = CI->getOperand(1);
+    if ((Op0 == Invariant && Op1 == Val) || (Op0 == Val && Op1 == Invariant)) {
+      LLVMContext &Ctx = Inst->getContext();
+      if (CI->getPredicate() == CmpInst::ICMP_EQ)
+        return ConstantInt::getFalse(Ctx);
+      else 
+        return ConstantInt::getTrue(Ctx);
+     }
+  }
+
+  // FIXME: there may be other opportunities, e.g. comparison with floating
+  // point, or Invariant - Val != 0, etc.
+  return nullptr;
+}

test/Transforms/LoopUnswitch/simplify-with-nonvalness.ll

@@ -0,0 +1,58 @@
+; RUN: opt < %s -loop-unswitch -verify-loop-info -S < %s 2>&1 | FileCheck %s
+
+; There are 1 case and 1 default case in the switch. after we unswitch, we know the
+; %a is definitely not 0 in one of the unswitched loop, make sure we take advantage
+; of that and simplify the branches in the loop.
+;
+; CHECK: define void @simplify_with_nonvalness(
+
+; This is the loop in which we know %a is definitely 0.
+; CHECK: sw.bb.us:
+; CHECK: br i1 true, label %if.then.us, label %if.end.us
+
+; This is the loop in which we do not know what %a is but we know %a is definitely NOT 0.
+; Make sure we use that information to simplify.
+; The icmp eq i32 %a, 0 in one of the unswitched loop is simplified to false.
+; CHECK: sw.bb.split:
+; CHECK: br i1 false, label %if.then, label %if.end
+
+define void @simplify_with_nonvalness(i32 %a) #0 {
+entry:
+  br label %for.cond
+
+for.cond:
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1024
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:
+  switch i32 %a, label %sw.default [
+    i32 0, label %sw.bb
+  ]
+
+sw.bb:
+  %cmp1 = icmp eq i32 %a, 0
+  br i1 %cmp1, label %if.then, label %if.end
+
+if.then:
+  call void (...) @bar()
+  br label %if.end
+
+if.end:
+  br label %sw.epilog
+
+sw.default:
+  br label %sw.epilog
+
+sw.epilog:
+  br label %for.inc
+
+for.inc:
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:
+  ret void
+}
+
+declare void @bar(...)