[InstCombine] Allow sinking from unique predecessor with multiple edges

[junbuml]

Summary

We can allow sinking if the single user block has only one unique predecessor, regardless of the number of edges. Note that a switch statement with multiple cases can have the same destination.

[reames]

LGTM

In general, I don't like the fact we have sinking logic in InstCombine at all, but this is a perfectly reasonable extension of the existing design.

llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp

if (I->hasOneUse()) {
           if (*SI == UserParent) {
             UserIsSuccessor = true;
             break;
           }
 
         // If the user is one of our immediate successors, and if that successor
         // only has us as a predecessors (we'd have to split the critical edge
         // otherwise), we can keep going.
-        if (UserIsSuccessor && UserParent->getSinglePredecessor()) {
+        if (UserIsSuccessor && UserParent->getUniquePredecessor()) {
           // Okay, the CFG is simple enough, try to sink this instruction.
           if (TryToSinkInstruction(I, UserParent)) {
             DEBUG(dbgs() << "IC: Sink: " << *I << '\n');
             MadeIRChange = true;
             // We'll add uses of the sunk instruction below, but since sinking
             // can expose opportunities for it's *operands* add them to the
             // worklist
             for (Use &U : I->operands())

llvm/trunk/test/Transforms/InstCombine/sink_instruction.ll

bb2:                                              ; preds = %bb, %bb1
   %exitcond = icmp eq i32 %4, 1000000             ; <i1> [#uses=1]
   br i1 %exitcond, label %bb4, label %bb
 
 bb4:                                              ; preds = %bb2
   ret i32 %x_addr.0
 }
 
 declare i32 @bar()
+
+define i32 @test3(i32* nocapture readonly %P, i32 %i) {
+entry:
+  %idxprom = sext i32 %i to i64
+  %arrayidx = getelementptr inbounds i32, i32* %P, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  switch i32 %i, label %sw.epilog [
+    i32 5, label %sw.bb
+    i32 2, label %sw.bb
+  ]
+
+sw.bb:                                            ; preds = %entry, %entry
+; CHECK-LABEL: sw.bb:
+; CHECK: %idxprom = sext i32 %i to i64
+; CHECK: %arrayidx = getelementptr inbounds i32, i32* %P, i64 %idxprom
+; CHECK: %0 = load i32, i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %i
+  br label %sw.epilog
+
+sw.epilog:                                        ; preds = %entry, %sw.bb
+  %sum.0 = phi i32 [ %add, %sw.bb ], [ 0, %entry ]
+  ret i32 %sum.0
+}