Revert 57dd4b0 "[ValueTracking] Allow context-sensitive nullness check for non-pointers"

This caused miscompiles of Chromium (https://crbug.com/1023818). The reduced
repro is small enough to fit here:

  $ cat /tmp/a.c
  unsigned char f(unsigned char *p) {
    unsigned char result = 0;
    for (int shift = 0; shift < 1; ++shift)
      result |= p[0] << (shift * 8);
    return result;
  }
  $ bin/clang -O2 -S -o - /tmp/a.c | grep -A4 f:
  f:                                      # @f
          .cfi_startproc
  # %bb.0:                                # %entry
          xorl    %eax, %eax
          retq

That's nicely optimized, but I don't think it's the right result :-)

> Same as D60846 but with a fix for the problem encountered there which
> was a missing context adjustment in the handling of PHI nodes.
>
> The test that caused D60846 to be reverted was added in e15ab8f277c7.
>
> Reviewers: nikic, nlopes, mkazantsev,spatel, dlrobertson, uabelho, hakzsam
>
> Subscribers: hiraditya, bollu, llvm-commits
>
> Tags: #llvm
>
> Differential Revision: https://reviews.llvm.org/D69571

This reverts commit 57dd4b03e4806bbb4760ab6150940150d884df20.

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index b26c6294dce..f03a4a6eee4 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1902,8 +1902,8 @@ static bool isGEPKnownNonNull(const GEPOperator *GEP, unsigned Depth,
 static bool isKnownNonNullFromDominatingCondition(const Value *V,
                                                   const Instruction *CtxI,
                                                   const DominatorTree *DT) {
-  if (isa<Constant>(V))
-    return false;
+  assert(V->getType()->isPointerTy() && "V must be pointer type");
+  assert(!isa<ConstantData>(V) && "Did not expect ConstantPointerNull");
 
   if (!CtxI || !DT)
     return false;
@@ -2078,11 +2078,12 @@ bool isKnownNonZero(const Value *V, unsigned Depth, const Query &Q) {
     }
   }
 
-  if (isKnownNonNullFromDominatingCondition(V, Q.CxtI, Q.DT))
-    return true;
 
   // Check for recursive pointer simplifications.
   if (V->getType()->isPointerTy()) {
+    if (isKnownNonNullFromDominatingCondition(V, Q.CxtI, Q.DT))
+      return true;
+
     // Look through bitcast operations, GEPs, and int2ptr instructions as they
     // do not alter the value, or at least not the nullness property of the
     // value, e.g., int2ptr is allowed to zero/sign extend the value.