[GlobalsAA] Partially back out r248576

See PR25822 for a more full summary, but we were conflating the concepts of "capture" and "escape". We were proving nocapture and using that proof to infer noescape, which is not true. Escaped-ness is a function-local property - as soon as a value is used in a call argument it escapes. Capturedness is a related but distinct property. It implies a *temporally limited* escape. Consider:

  static int a;
  int b;
  int g(int * nocapture arg);
  int f() {
    a = 2;  // Even though a escapes to g, it is not captured so can be treated as non-escaping here.
    g(&a);  // But here it must be treated as escaping.
    g(&b);  // Now that g(&a) has returned we know it was not captured so we can treat it as non-escaping again.
  }

The original commit did not sufficiently understand this nuance and so caused PR25822 and PR26046.

r248576 included both a performance improvement (which has been backed out) and a related conformance fix (which has been kept along with its testcase).

llvm-svn: 257058

1 files changed, 0 insertions, 15 deletions

diff --git a/llvm/lib/Analysis/GlobalsModRef.cpp b/llvm/lib/Analysis/GlobalsModRef.cpp
index 249f3954d55..1babb822074 100644
--- a/llvm/lib/Analysis/GlobalsModRef.cpp
+++ b/llvm/lib/Analysis/GlobalsModRef.cpp
@@ -358,21 +358,6 @@ bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
         if (CS.isArgOperand(&U) && isFreeCall(I, &TLI)) {
           if (Writers)
             Writers->insert(CS->getParent()->getParent());
-        } else if (CS.doesNotCapture(CS.getDataOperandNo(&U))) {
-          Function *ParentF = CS->getParent()->getParent();
-          // A nocapture argument may be read from or written to, but does not
-          // escape unless the call can somehow recurse.
-          //
-          // nocapture "indicates that the callee does not make any copies of
-          // the pointer that outlive itself". Therefore if we directly or
-          // indirectly recurse, we must treat the pointer as escaping.
-          if (FunctionToSCCMap[ParentF] ==
-              FunctionToSCCMap[CS.getCalledFunction()])
-            return true;
-          if (Readers)
-            Readers->insert(ParentF);
-          if (Writers)
-            Writers->insert(ParentF);
         } else {
           return true; // Argument of an unknown call.
         }