[ValueTracking] Let propagatesPoison support binops/unaryops/cast/etc.

[aqjune]

Summary

This patch makes propagatesPoison be more accurate by returning true on
more bin ops/unary ops/casts/etc.

The changed test in ScalarEvolution/nsw.ll was introduced by
https://github.com/llvm/llvm-project/commit/a19edc4d15b0dae0210b90615775edd76f021008 .
IIUC, the goal of the tests is to show that iv.inc's SCEV expression still has
no-overflow flags even if the loop isn't in the wanted form.
It becomes more accurate with this patch, so think this is okay.

[jdoerfert]

llvm/lib/Transforms/Instrumentation/PoisonChecking.cpp

348

I think this makes sense but we need to get input from others on this.

llvm/test/Analysis/ScalarEvolution/nsw.ll

242

Please verify as the function name suggests there is some problem here. I noted in the previous patch that I think this is correct but we should check and potentially change the function name or record our findings.

[nikic]

llvm/include/llvm/Analysis/ValueTracking.h

567

As I mentioned on the other review, I don't think this is the best choice for this API. propagatesPoison should return true as much as possible for best results. It can return true for immediate UB, so imho it should. At least I don't see in which situation it would be advantageous to return false here.

For example, right now propagatesPoison on udiv returns false, because poison op2 results in IUB. However, poison op1 will propagate poison. Unless you want to distinguish which operand is poison, it would be better to always return true.

(Though distinguishing which operand is poison may be necessary for more accurate select handling, which propagates poison only on op1.)

llvm/lib/Transforms/Instrumentation/PoisonChecking.cpp

348

Lang ref does kind of state this by omission:

Values other than phi nodes and select instructions depend on their operands.

We have select -> and/or transforms that are known to be unsound if and/or do not block poison, but work is underway to fix those.

[aqjune]

llvm/include/llvm/Analysis/ValueTracking.h

567

Okay, so it requires the users of propagatesPoison to specially deal with div/rem operations. We have getGuaranteedNonPoisonOp, which can be used to leave poison-propagating operands only. I think the concern makes sense.

llvm/lib/Transforms/Instrumentation/PoisonChecking.cpp

348

The sentence of LangRef implicitly describes the semantics of and/or, as @nikic says, but I have an impression that the semantics of and/or frequently has been controversial.

I remember that every possible other candidates for and/or was problematic because it breaks many optimizations on logical <-> arithmetic ops.

I'll bring concrete examples for these by running LLVM unit tests with modified semantics of and/or with Alive2 in a few days. After this issue is resolved, it will be great if we can explicitly state in LangRef why and/or should propagate poison regardless of masking values.

[aqjune]

• propagatesPoison returns true on div/rem

[aqjune]

• Write formal specification of propagatesPoison

[aqjune]

Hello all,
I ran experiment with an alternative semantics which defines and poison, 0 and or poison, -1 to block propagation of poison. In this semantics, and poison, 0 is 0, and or poison, -1 is -1.

Here are a few optimizations that I discovered from llvm/test/Transforms becoming incorrect in the alternative semantics:

1. <Transforms/InstCombine/and-or-icmps.ll>

define i1 @PR2330(i32 %a, i32 %b) {
%0:
  %cmp1 = icmp ult i32 %a, 8
  %cmp2 = icmp ult i32 %b, 8
  %and = and i1 %cmp2, %cmp1
  ret i1 %and
}
=>
define i1 @PR2330(i32 %a, i32 %b) {
%0:
  %1 = or i32 %b, %a
  %2 = icmp ult i32 %1, 8
  ret i1 %2
}

If %a = poison && %b = 9, the source returns poison && 0 = 0, whereas target returns poison.

2. <Transforms/InstCombine/apint-shift-simplify.ll>

define i41 @test0(i41 %A, i41 %B, i41 %C) {
%0:
  %X = shl i41 %A, %C
  %Y = shl i41 %B, %C
  %Z = and i41 %X, %Y
  ret i41 %Z
}
=>
define i41 @test0(i41 %A, i41 %B, i41 %C) {
%0:
  %X1 = and i41 %A, %B
  %Z = shl i41 %X1, %C
  ret i41 %Z
}

If %A = 100...00 && %B = poison && %C = 1, the source returns 0 whereas target returns poison.

3. <Transforms/InstCombine/and-fcmp.ll>

define i1 @PR1738(double %x, double %y) {
%0:
  %cmp1 = fcmp ord %x, 0.000000
  %cmp2 = fcmp ord %y, 0.000000
  %and = and i1 %cmp1, %cmp2
  ret i1 %and 
}
=>
define i1 @PR1738(double %x, double %y) {
%0:
  %1 = fcmp ord %x, %y
  ret i1 %1
}

If %x = poison && %y = NaN, the source returns 0 whereas target returns poison.

When I run Alive2, I see that 50 more lit tests from test/Transforms/ are failing, compared with the base semantics (and/ors unconditionally propagating poison)
To fix this - we need to redefine the semantics of many existing operations. For example, fcmp ord should be redefined to determine when it returns poison / when it does not, which might be a pain. Same for shl/icmp etc. So I vote for the simpler semantics (and/or unconditionally propagating poison).

Also, LangRef states that values other than select/phi depends on their operands, and an instruction that depends on a poison value, produces a poison value, so it implies that and/or propagates poison.
propagatesPoison in this patch returns true on and/or operations for this reason.

llvm/test/Analysis/ScalarEvolution/nsw.ll

242

I investigated a bit, and the history was like this:

https://github.com/llvm/llvm-project/commit/7e4a64167d4d2e7b0b680fae1706182223047af1 fixed a bug in SCEV which was incorrectly adding no-wrap flag to a post-inc add recurrence. The patch added isAddRecNeverPoison, which checks whether it is UB if the given add recurrence is poison, by consecutively calling propagatesFullPoison on its use chain.

However, the patch wasn't calling propagatesFullPoison on the direct uses of the add recurrence, so https://github.com/llvm/llvm-project/commit/a19edc4d15b0dae0210b90615775edd76f021008 added the call & added these two tests (bad_postinc_nsw_a/b).

In bad_postinc_nsw_b, and %iv.inc, 0 was inserted to test whether the first propagateFullPoison check successfully blocks propagation of poison. Now propagatesPoison returns true, so the test has changed.
To summarize, the validity of this change depends on whether and x, 0 should propagate poison or not. I think it should propagate (as I wrote on another comment). As suggested, I'll change the function name & leave a comment.

[aqjune]

Update a comment & clang-format

[aqjune]

Added people who might be interested in this issue as subscribers.

[nikic]

Thanks for running the alive experiment. The results look pretty compelling and are in line with what I'd expect. Giving one instruction special poison semantics has a snowball effect that will quickly lead us to undef-like reasoning.

This LG to me, but please wait for a second opinion.

llvm/lib/Analysis/ValueTracking.cpp

4852

Maybe list call/invoke explicitly here?

[aqjune]

Make call / invoke explicit

[spatel]

LGTM - see inline for some grammar nits.

llvm/include/llvm/Analysis/ValueTracking.h

565–570

raise -> raises

569

the operand -> operands

llvm/lib/Analysis/ValueTracking.cpp

4862

Add period to end of sentence.

[aqjune]

Fix grammar errors

[aqjune]

llvm/test/Analysis/ScalarEvolution/nsw.ll

242

Does this address your concern? @jdoerfert

[reames]

llvm/include/llvm/Analysis/ValueTracking.h

567

I really don't think that having UB operations return true is the right semantic here. It doesn't add any value. If we have a UB operation with provable poison, then we've missed the opportunity to prune that path. Having some later optimization trigger (which is what the proposed semantic does), feels like side stepping the problem.

(I'm just expressing an opinion, not blocking the patch. I'm fine with this going in, we can continue the discussion and change our minds later if my view turns out to convince others.)

[nlopes]

llvm/include/llvm/Analysis/ValueTracking.h

567

I guess it's still useful to know that udiv propagates poison regardless whether we know it is UB or not.
Most of the times we won't know if an operation triggers UB or not, so it feels right to at least some information. We have other APIs to check whether an instruction triggers UB. You are right in that we are pushing a bit more responsibility to the user of this analysis (for perf reasons, not correctness), but for the reasons I've stated above, I think it's a good tradeoff.
I'm in favor of the patch as-is.

[aqjune]

llvm/test/Analysis/ScalarEvolution/nsw.ll

242

ping @jdoerfert

[jdoerfert]

llvm/test/Analysis/ScalarEvolution/nsw.ll

242

OK with me.

[aqjune]

Thanks all, I landed this patch.
For the update in LangRef, I checked that it already has this specific example:
%still_poison = and i32 %poison, 0 ; 0, but also poison.,
Making LLVM's implementation consistent with LangRef through this patch seems enough.

llvm/include/llvm/Analysis/ValueTracking.h

 //===- llvm/Analysis/ValueTracking.h - Walk computations --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
class Value;
 
   /// Return true if this function can prove that the instruction I
   /// is executed for every iteration of the loop L.
   ///
   /// Note that this currently only considers the loop header.
   bool isGuaranteedToExecuteForEveryIteration(const Instruction *I,
                                               const Loop *L);
 
-  /// Return true if this function can prove that I is guaranteed to yield
-  /// poison if at least one of its operands is poison.
+  /// Return true if I yields poison or raise UB if any of its operands is
+  /// poison.
+  /// Formally, given I = `r = op v1 v2 .. vN`, propagatesPoison returns true
+  /// if, for all i, r is evaluated to poison or op raises UB if vi = poison.
+  /// To filter out the operand that raise UB on poison, you can use
+  /// getGuaranteedNonPoisonOp.
   bool propagatesPoison(const Instruction *I);
 
   /// Return either nullptr or an operand of I such that I will trigger
   /// undefined behavior if I is executed and that operand has a poison
   /// value.
   const Value *getGuaranteedNonPoisonOp(const Instruction *I);
 
   /// Return true if the given instruction must trigger undefined behavior.

llvm/lib/Analysis/ValueTracking.cpp

 //===- ValueTracking.cpp - Walk computations to compute properties --------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
bool llvm::isGuaranteedToExecuteForEveryIteration(const Instruction *I,
   for (const Instruction &LI : *L->getHeader()) {
     if (&LI == I) return true;
     if (!isGuaranteedToTransferExecutionToSuccessor(&LI)) return false;
   }
   llvm_unreachable("Instruction not contained in its own parent basic block.");
 }
 
 bool llvm::propagatesPoison(const Instruction *I) {
-  // TODO: This should include all instructions apart from phis, selects and
-  // call-like instructions.
   switch (I->getOpcode()) {
-  case Instruction::Add:
-  case Instruction::Sub:
-  case Instruction::Xor:
-  case Instruction::Trunc:
-  case Instruction::BitCast:
-  case Instruction::AddrSpaceCast:
-  case Instruction::Mul:
-  case Instruction::Shl:
+  case Instruction::Freeze:
+  case Instruction::Select:
+  case Instruction::PHI:
+    return false;
+  case Instruction::ICmp:
+  case Instruction::FCmp:
   case Instruction::GetElementPtr:
-    // These operations all propagate poison unconditionally. Note that poison
-    // is not any particular value, so xor or subtraction of poison with
-    // itself still yields poison, not zero.
-    return true;
-
-  case Instruction::AShr:
-  case Instruction::SExt:
-    // For these operations, one bit of the input is replicated across
-    // multiple output bits. A replicated poison bit is still poison.
     return true;
-
-  case Instruction::ICmp:
-    // Comparing poison with any value yields poison.  This is why, for
-    // instance, x s< (x +nsw 1) can be folded to true.
+  default:
+    if (isa<BinaryOperator>(I) || isa<UnaryOperator>(I) || isa<CastInst>(I))
       return true;
 
-  default:
+    // Be conservative and return false
     return false;
   }
 }
 
 const Value *llvm::getGuaranteedNonPoisonOp(const Instruction *I) {
   switch (I->getOpcode()) {
     case Instruction::Store:
       return cast<StoreInst>(I)->getPointerOperand();

llvm/lib/Transforms/Instrumentation/PoisonChecking.cpp

 //===- PoisonChecking.cpp - -----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Implements a transform pass which instruments IR such that poison semantics
 // are made explicit.  That is, it provides a (possibly partial) executable
 // semantics for every instruction w.r.t. poison as specified in the LLVM
 // LangRef.  There are obvious parallels to the sanitizer tools, but this pass
 // is focused purely on the semantics of LLVM IR, not any particular source
 // language.   If you're looking for something to see if your C/C++ contains
-// UB, this is not it.  
-// 
+// UB, this is not it.
+//
 // The rewritten semantics of each instruction will include the following
-// components: 
+// components:
 //
 // 1) The original instruction, unmodified.
 // 2) A propagation rule which translates dynamic information about the poison
 //    state of each input to whether the dynamic output of the instruction
 //    produces poison.
 // 3) A creation rule which validates any poison producing flags on the
 //    instruction itself (e.g. checks for overflow on nsw).
 // 4) A check rule which traps (to a handler function) if this instruction must
 //    execute undefined behavior given the poison state of it's inputs.
 //
 // This is a must analysis based transform; that is, the resulting code may
 // produce a false negative result (not report UB when actually exists
 // according to the LangRef spec), but should never produce a false positive
 // (report UB where it doesn't exist).
 //
 // Use cases for this pass include:
 // - Understanding (and testing!) the implications of the definition of poison
 //   from the LangRef.
 // - Validating the output of a IR fuzzer to ensure that all programs produced
 //   are well defined on the specific input used.
 // - Finding/confirming poison specific miscompiles by checking the poison
 //   status of an input/IR pair is the same before and after an optimization
-//   transform. 
+//   transform.
 // - Checking that a bugpoint reduction does not introduce UB which didn't
 //   exist in the original program being reduced.
 //
 // The major sources of inaccuracy are currently:
 // - Most validation rules not yet implemented for instructions with poison
 //   relavant flags.  At the moment, only nsw/nuw on add/sub are supported.
 // - UB which is control dependent on a branch on poison is not yet
 //   reported. Currently, only data flow dependence is modeled.
 // - Poison which is propagated through memory is not modeled.  As such,
 //   storing poison to memory and then reloading it will cause a false negative
 //   as we consider the reloaded value to not be poisoned.
 // - Poison propagation across function boundaries is not modeled.  At the
 //   moment, all arguments and return values are assumed not to be poison.
 // - Undef is not modeled.  In particular, the optimizer's freedom to pick
 //   concrete values for undef bits so as to maximize potential for producing
-//   poison is not modeled.  
+//   poison is not modeled.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Instrumentation/PoisonChecking.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/ValueTracking.h"
for (; i < Ops.size(); i++)
     if (!isConstantFalse(Ops[i]))
       Accum = B.CreateOr(Accum, Ops[i]);
   return Accum;
 }
 
 static void generateCreationChecksForBinOp(Instruction &I,
                                            SmallVectorImpl<Value*> &Checks) {
   assert(isa<BinaryOperator>(I));
-  
   IRBuilder<> B(&I);
   Value *LHS = I.getOperand(0);
   Value *RHS = I.getOperand(1);
   switch (I.getOpcode()) {
   default:
     return;
   case Instruction::Add: {
     if (I.hasNoSignedWrap()) {

 static bool rewrite(Function &F) {
   auto * const Int1Ty = Type::getInt1Ty(F.getContext());
 
   DenseMap<Value *, Value *> ValToPoison;
 
   for (BasicBlock &BB : F)
     for (auto I = BB.begin(); isa<PHINode>(&*I); I++) {
       auto *OldPHI = cast<PHINode>(&*I);
-      auto *NewPHI = PHINode::Create(Int1Ty, 
+      auto *NewPHI = PHINode::Create(Int1Ty,
                                      OldPHI->getNumIncomingValues());
       for (unsigned i = 0; i < OldPHI->getNumIncomingValues(); i++)
         NewPHI->addIncoming(UndefValue::get(Int1Ty),
                             OldPHI->getIncomingBlock(i));
       NewPHI->insertBefore(OldPHI);
       ValToPoison[OldPHI] = NewPHI;
     }
-  
   for (BasicBlock &BB : F)
     for (Instruction &I : BB) {
       if (isa<PHINode>(I)) continue;
 
       IRBuilder<> B(cast<Instruction>(&I));
-      
       // Note: There are many more sources of documented UB, but this pass only
       // attempts to find UB triggered by propagation of poison.
       if (Value *Op = const_cast<Value*>(getGuaranteedNonPoisonOp(&I)))
         CreateAssertNot(B, getPoisonFor(ValToPoison, Op));
 
       if (LocalCheck)
         if (auto *RI = dyn_cast<ReturnInst>(&I))
           if (RI->getNumOperands() != 0) {
PreservedAnalyses PoisonCheckingPass::run(Module &M,
   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
 }
 
 PreservedAnalyses PoisonCheckingPass::run(Function &F,
                                           FunctionAnalysisManager &AM) {
   return rewrite(F) ? PreservedAnalyses::none() : PreservedAnalyses::all();
 }
 
 
 /* Major TODO Items:
    - Control dependent poison UB
    - Strict mode - (i.e. must analyze every operand)
      - Poison through memory
      - Function ABIs
      - Full coverage of intrinsics, etc.. (ouch)
 
    Instructions w/Unclear Semantics:
    - shufflevector - It would seem reasonable for an out of bounds mask element
-     to produce poison, but the LangRef does not state.  
-   - and/or - It would seem reasonable for poison to propagate from both
-     arguments, but LangRef doesn't state and propagatesPoison doesn't
-     include these two.
+     to produce poison, but the LangRef does not state.
    - all binary ops w/vector operands - The likely interpretation would be that
      any element overflowing should produce poison for the entire result, but
      the LangRef does not state.
    - Floating point binary ops w/fmf flags other than (nnan, noinfs).  It seems
      strange that only certian flags should be documented as producing poison.
 
    Cases of clear poison semantics not yet implemented:
    - Exact flags on ashr/lshr produce poison
    - NSW/NUW flags on shl produce poison
    - Inbounds flag on getelementptr produce poison
    - fptosi/fptoui (out of bounds input) produce poison
    - Scalable vector types for insertelement/extractelement
    - Floating point binary ops w/fmf nnan/noinfs flags produce poison
  */

llvm/test/Analysis/ScalarEvolution/nsw.ll

 entry:
   br label %loop
 
 loop:
   %iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
   %iv.inc = add nsw i32 %iv, 7
   %iv.inc.and = and i32 %iv.inc, 0
 ; CHECK:    %iv.inc = add nsw i32 %iv, 7
-; CHECK-NEXT:  -->  {7,+,7}<nuw><%loop>
+; CHECK-NEXT:  -->  {7,+,7}<nuw><nsw><%loop>
   %becond = icmp ult i32 %iv.inc.and, %n
   br i1 %becond, label %loop, label %leave
 
 leave:
   ret void
 }
 
 declare void @may_exit() nounwind
 
 define void @pr28012(i32 %n) {
 ; CHECK-LABEL: Classifying expressions for: @pr28012
 entry:
   br label %loop
 

llvm/unittests/Analysis/ValueTrackingTest.cpp

 //===- ValueTrackingTest.cpp - ValueTracking tests ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
parseAssembly(
       "define <2 x i32> @test(<2 x i1> %x) {\n"
       "  %sext = sext <2 x i1> %x to <2 x i32>\n"
       "  %A = shufflevector <2 x i32> %sext, <2 x i32> undef, <2 x i32> <i32 0, i32 2>\n"
       "  ret <2 x i32> %A\n"
       "}\n");
   EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 1u);
 }
 
+TEST(ValueTracking, propagatesPoison) {
+  std::string AsmHead =
+      "declare i32 @g(i32)\n"
+      "define void @f(i32 %x, i32 %y, float %fx, float %fy, i1 %cond, i8* %p) {\n";
+  std::string AsmTail = "  ret void\n}";
+  // (propagates poison?, IR instruction)
+  SmallVector<std::pair<bool, std::string>, 32> Data = {
+      {true, "add i32 %x, %y"},
+      {true, "add nsw nuw i32 %x, %y"},
+      {true, "ashr i32 %x, %y"},
+      {true, "lshr exact i32 %x, 31"},
+      {true, "fcmp oeq float %fx, %fy"},
+      {true, "icmp eq i32 %x, %y"},
+      {true, "getelementptr i8, i8* %p, i32 %x"},
+      {true, "getelementptr inbounds i8, i8* %p, i32 %x"},
+      {true, "bitcast float %fx to i32"},
+      {false, "select i1 %cond, i32 %x, i32 %y"},
+      {false, "freeze i32 %x"},
+      {true, "udiv i32 %x, %y"},
+      {true, "urem i32 %x, %y"},
+      {true, "sdiv exact i32 %x, %y"},
+      {true, "srem i32 %x, %y"},
+      {false, "call i32 @g(i32 %x)"}};
+
+  std::string AssemblyStr = AsmHead;
+  for (auto &Itm : Data)
+    AssemblyStr += Itm.second + "\n";
+  AssemblyStr += AsmTail;
+
+  LLVMContext Context;
+  SMDiagnostic Error;
+  auto M = parseAssemblyString(AssemblyStr, Error, Context);
+  assert(M && "Bad assembly?");
+
+  auto *F = M->getFunction("f");
+  assert(F && "Bad assembly?");
+
+  auto &BB = F->getEntryBlock();
+
+  int Index = 0;
+  for (auto &I : BB) {
+    if (isa<ReturnInst>(&I))
+      break;
+    EXPECT_EQ(propagatesPoison(&I), Data[Index].first)
+        << "Incorrect answer at instruction " << Index << " = " << I;
+    Index++;
+  }
+}
+
 TEST(ValueTracking, canCreatePoison) {
   std::string AsmHead =
       "declare i32 @g(i32)\n"
       "define void @f(i32 %x, i32 %y, float %fx, float %fy, i1 %cond, "
       "<4 x i32> %vx, <4 x i32> %vx2, <vscale x 4 x i32> %svx, i8* %p) {\n";
   std::string AsmTail = "  ret void\n}";
   // (can create poison?, IR instruction)
   SmallVector<std::pair<bool, std::string>, 32> Data = {