Teach jump threading to "look through" a select when the branch direction of a terminator depends on it.

When it sees a promising select it now tries to figure out whether the condition of the select is known in any of the predecessors and if so it maps the operands appropriately.

llvm-svn: 121859

lib/Transforms/Scalar/JumpThreading.cpp

@@ -538,6 +538,40 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
     }
   }
 
+  if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
+    // Handle select instructions where at least one operand is a known constant
+    // and we can figure out the condition value for any predecessor block.
+    Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
+    Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
+    PredValueInfoTy Conds;
+    if ((TrueVal || FalseVal) &&
+        ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
+                                        WantInteger)) {
+      for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
+        Constant *Cond = Conds[i].first;
+
+        // Figure out what value to use for the condition.
+        bool KnownCond;
+        if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
+          // A known boolean.
+          KnownCond = CI->isOne();
+        } else {
+          assert(isa<UndefValue>(Cond) && "Unexpected condition value");
+          // Either operand will do, so be sure to pick the one that's a known
+          // constant.
+          // FIXME: Do this more cleverly if both values are known constants?
+          KnownCond = (TrueVal != 0);
+        }
+
+        // See if the select has a known constant value for this predecessor.
+        if (Constant *Val = KnownCond ? TrueVal : FalseVal)
+          Result.push_back(std::make_pair(Val, Conds[i].second));
+      }
+
+      return !Result.empty();
+    }
+  }
+
   // If all else fails, see if LVI can figure out a constant value for us.
   Constant *CI = LVI->getConstant(V, BB);
   if (Constant *KC = getKnownConstant(CI, Preference)) {

test/Transforms/JumpThreading/select.ll

@@ -0,0 +1,123 @@
+; RUN: opt -S -jump-threading < %s | FileCheck %s
+
+declare void @foo()
+declare void @bar()
+declare void @baz()
+declare void @quux()
+
+
+; Jump threading of branch with select as condition.
+; Mostly theoretical since instruction combining simplifies all selects of
+; booleans where at least one operand is true/false/undef.
+
+; CHECK: @test_br
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %cond, label %L1,
+define void @test_br(i1 %cond, i1 %value) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L3
+L0:
+  %expr = select i1 %cond, i1 true, i1 %value
+  br i1 %expr, label %L1, label %L2
+
+L1:
+  call void @foo()
+  ret void
+L2:
+  call void @bar()
+  ret void
+L3:
+  call void @baz()
+  br label %L0
+}
+
+
+; Jump threading of switch with select as condition.
+
+; CHECK: @test_switch
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %cond, label %L1,
+define void @test_switch(i1 %cond, i8 %value) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L4
+L0:
+  %expr = select i1 %cond, i8 1, i8 %value
+  switch i8 %expr, label %L3 [i8 1, label %L1 i8 2, label %L2]
+
+L1:
+  call void @foo()
+  ret void
+L2:
+  call void @bar()
+  ret void
+L3:
+  call void @baz()
+  ret void
+L4:
+  call void @quux()
+  br label %L0
+}
+
+; Make sure the blocks in the indirectbr test aren't trivially removable as
+; successors by taking their addresses.
+@anchor = constant [3 x i8*] [
+  i8* blockaddress(@test_indirectbr, %L1),
+  i8* blockaddress(@test_indirectbr, %L2),
+  i8* blockaddress(@test_indirectbr, %L3)
+]
+
+
+; Jump threading of indirectbr with select as address.
+
+; CHECK: @test_indirectbr
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %cond, label %L1, label %L3
+define void @test_indirectbr(i1 %cond, i8* %address) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L3
+L0:
+  %indirect.goto.dest = select i1 %cond, i8* blockaddress(@test_indirectbr, %L1), i8* %address
+  indirectbr i8* %indirect.goto.dest, [label %L1, label %L2, label %L3]
+
+L1:
+  call void @foo()
+  ret void
+L2:
+  call void @bar()
+  ret void
+L3:
+  call void @baz()
+  ret void
+}
+
+
+; A more complicated case: the condition is a select based on a comparison.
+
+; CHECK: @test_switch_cmp
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %cond, label %L0, label %[[THREADED:[A-Za-z.0-9]+]]
+; CHECK: [[THREADED]]:
+; CHECK-NEXT: call void @quux
+; CHECK-NEXT: br label %L1
+define void @test_switch_cmp(i1 %cond, i32 %val, i8 %value) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L4
+L0:
+  %val.phi = phi i32 [%val, %entry], [-1, %L4]
+  %cmp = icmp slt i32 %val.phi, 0
+  %expr = select i1 %cmp, i8 1, i8 %value
+  switch i8 %expr, label %L3 [i8 1, label %L1 i8 2, label %L2]
+
+L1:
+  call void @foo()
+  ret void
+L2:
+  call void @bar()
+  ret void
+L3:
+  call void @baz()
+  ret void
+L4:
+  call void @quux()
+  br label %L0
+}