SCEV: Don't assert about non-SCEV-able value in isSCEVExprNeverPoison() (PR28932)

[hans]

Summary

SCEV would assert for the test case at https://llvm.org/bugs/show_bug.cgi?id=28932#c3

I'm not familiar with this code, but perhaps we can just bail out here?

Please take a look. If this is easy to fix, I'd like it for 3.9.

[hans]

Fix comment.

[sanjoy]

The bailout lgtm for now. Can you also add a TODO stating that we can do better here in some cases?

lib/Analysis/ScalarEvolution.cpp

4869 ↗

(On Diff #68438)

Add a one-liner here that I could be an extractvalue from a call to an overflow intrinsic, since it is somewhat non-obvious why you'd get a non-SCEVable value here.

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

bool ScalarEvolution::isSCEVExprNeverPoison(const Instruction *I) {
   // every iteration of that loop. That implies that the value that I
   // calculates does not wrap anywhere in the loop, so then we can apply the
   // flags to the SCEV.
   //
   // We check isLoopInvariant to disambiguate in case we are adding recurrences
   // from different loops, so that we know which loop to prove that I is
   // executed in.
   for (unsigned OpIndex = 0; OpIndex < I->getNumOperands(); ++OpIndex) {
+    // I could be an extractvalue from a call to an overflow intrinsic.
+    // TODO: We can do better here in some cases.
+    if (!isSCEVable(I->getOperand(OpIndex)->getType()))
+      return false;
     const SCEV *Op = getSCEV(I->getOperand(OpIndex));
     if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(Op)) {
       bool AllOtherOpsLoopInvariant = true;
       for (unsigned OtherOpIndex = 0; OtherOpIndex < I->getNumOperands();
            ++OtherOpIndex) {
         if (OtherOpIndex != OpIndex) {
           const SCEV *OtherOp = getSCEV(I->getOperand(OtherOpIndex));
           if (!isLoopInvariant(OtherOp, AddRec->getLoop())) {

llvm/trunk/test/Analysis/ScalarEvolution/flags-from-poison.ll

 
 outer.be:
   %cond3 = icmp eq i32 %o_idx, %outer_l
   br i1 %cond3, label %exit, label %outer
 
 exit:
   ret void
 }
+
+
+; PR28932: Don't assert on non-SCEV-able value %2.
+%struct.anon = type { i8* }
+@a = common global %struct.anon* null, align 8
+@b = common global i32 0, align 4
+declare { i32, i1 } @llvm.ssub.with.overflow.i32(i32, i32)
+declare void @llvm.trap()
+define i32 @pr28932() {
+entry:
+  %.pre = load %struct.anon*, %struct.anon** @a, align 8
+  %.pre7 = load i32, i32* @b, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %cont6, %entry
+  %0 = phi i32 [ %3, %cont6 ], [ %.pre7, %entry ]
+  %1 = phi %struct.anon* [ %.ph, %cont6 ], [ %.pre, %entry ]
+  %tobool = icmp eq %struct.anon* %1, null
+  %2 = tail call { i32, i1 } @llvm.ssub.with.overflow.i32(i32 %0, i32 1)
+  %3 = extractvalue { i32, i1 } %2, 0
+  %4 = extractvalue { i32, i1 } %2, 1
+  %idxprom = sext i32 %3 to i64
+  %5 = getelementptr inbounds %struct.anon, %struct.anon* %1, i64 0, i32 0
+  %6 = load i8*, i8** %5, align 8
+  %7 = getelementptr inbounds i8, i8* %6, i64 %idxprom
+  %8 = load i8, i8* %7, align 1
+  br i1 %tobool, label %if.else, label %if.then
+
+if.then:                                          ; preds = %for.cond
+  br i1 %4, label %trap, label %cont6
+
+trap:                                             ; preds = %if.else, %if.then
+  tail call void @llvm.trap()
+  unreachable
+
+if.else:                                          ; preds = %for.cond
+  br i1 %4, label %trap, label %cont1
+
+cont1:                                            ; preds = %if.else
+  %conv5 = sext i8 %8 to i64
+  %9 = inttoptr i64 %conv5 to %struct.anon*
+  store %struct.anon* %9, %struct.anon** @a, align 8
+  br label %cont6
+
+cont6:                                            ; preds = %cont1, %if.then
+  %.ph = phi %struct.anon* [ %9, %cont1 ], [ %1, %if.then ]
+  store i32 %3, i32* @b, align 4
+  br label %for.cond
+}