From 6a8908e97960a3929876e52fe57a46982230bb03 Mon Sep 17 00:00:00 2001
From: James Molloy <james.molloy@arm.com>
Date: Thu, 7 Jan 2016 13:33:28 +0000
Subject: [PATCH] [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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257058 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/GlobalsModRef.cpp           | 15 -------
 test/Analysis/GlobalsModRef/nocapture.ll | 57 ------------------------
 2 files changed, 72 deletions(-)
 delete mode 100644 test/Analysis/GlobalsModRef/nocapture.ll

diff --git a/lib/Analysis/GlobalsModRef.cpp b/lib/Analysis/GlobalsModRef.cpp
index 249f3954d55..1babb822074 100644
--- a/lib/Analysis/GlobalsModRef.cpp
+++ b/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.
         }
diff --git a/test/Analysis/GlobalsModRef/nocapture.ll b/test/Analysis/GlobalsModRef/nocapture.ll
deleted file mode 100644
index 0cb80a10f8d..00000000000
--- a/test/Analysis/GlobalsModRef/nocapture.ll
+++ /dev/null
@@ -1,57 +0,0 @@
-; RUN: opt < %s -globals-aa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
-
-target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
-target triple = "x86_64-apple-macosx10.10.0"
-
-@a = internal global i32 0, align 4
-@b = internal global i32 0, align 4
-
-define void @g(i32* %p, void (i32*)* nocapture %ptr) {
-entry:
-  tail call void %ptr(i32* %p) #1
-  ret void
-}
-
-; CHECK-LABEL: Function: f
-; CHECK: MayAlias: i32* %p, i32* @a
-; CHECK: MayAlias: i32* %q, i32* @a
-define i32 @f(i32 %n, i32* nocapture readonly %p, i32* nocapture %q, void (i32*)* nocapture %ptr) {
-entry:
-  tail call void @g(i32* nonnull @a, void (i32*)* %ptr)
-  %arrayidx = getelementptr inbounds i32, i32* %p, i64 0
-  %z1 = load i32, i32* %arrayidx, align 4
-  %z2 = load i32, i32* %q, align 4
-  %add = add nsw i32 %z2, %z1
-  store i32 %add, i32* %q, align 4
-  ret i32 4
-}
-
-define void @g2(i32* nocapture %p, void (i32*)* nocapture %ptr) {
-entry:
-  tail call void %ptr(i32* %p) #1
-  ret void
-}
-
-; CHECK-LABEL: Function: f2
-; CHECK: NoAlias: i32* %p, i32* @b
-; CHECK: NoAlias: i32* %q, i32* @b
-define i32 @f2(i32 %n, i32* nocapture readonly %p, i32* nocapture %q, void (i32*)* nocapture %ptr) {
-entry:
-  tail call void @g2(i32* nonnull @b, void (i32*)* %ptr)
-  %arrayidx = getelementptr inbounds i32, i32* %p, i64 0
-  %z1 = load i32, i32* %arrayidx, align 4
-  %z2 = load i32, i32* %q, align 4
-  %add = add nsw i32 %z2, %z1
-  store i32 %add, i32* %q, align 4
-  ret i32 4
-}
-
-declare void @g3()
-
-; CHECK-LABEL: Function: f3
-; CHECK: NoAlias: i32* %p, i32* @b
-define void @f3(i32* nocapture readonly %p) {
-entry:
-  tail call void @g3() [ "deopt"(i32* @b, i32 *%p) ]
-  unreachable
-}
-- 
2.34.1