[PATCH] D60047: [CaptureTracking] Don't let comparisons against null escape inbounds pointers

[Ayke via Phabricator via llvm-commits]

aykevl updated this revision to Diff 200152.
aykevl marked an inline comment as done.
aykevl added a comment.

- replace some types with `auto`
- update comment for `getelementptr inbounds`


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D60047/new/

https://reviews.llvm.org/D60047

Files:
  lib/Analysis/CaptureTracking.cpp
  test/Transforms/FunctionAttrs/nocapture.ll


Index: test/Transforms/FunctionAttrs/nocapture.ll
===================================================================
--- test/Transforms/FunctionAttrs/nocapture.ll
+++ test/Transforms/FunctionAttrs/nocapture.ll
@@ -253,5 +253,26 @@
   ret void
 }
 
+; CHECK: define i1 @captureICmp(i32* readnone %x)
+define i1 @captureICmp(i32* %x) {
+  %1 = icmp eq i32* %x, null
+  ret i1 %1
+}
+
+; CHECK: define i1 @nocaptureInboundsGEPICmp(i32* nocapture readnone %x)
+define i1 @nocaptureInboundsGEPICmp(i32* %x) {
+  %1 = getelementptr inbounds i32, i32* %x, i32 5
+  %2 = bitcast i32* %1 to i8*
+  %3 = icmp eq i8* %2, null
+  ret i1 %3
+}
+
+; CHECK: define i1 @nocaptureDereferenceableOrNullICmp(i32* nocapture readnone dereferenceable_or_null(4) %x)
+define i1 @nocaptureDereferenceableOrNullICmp(i32* dereferenceable_or_null(4) %x) {
+  %1 = bitcast i32* %x to i8*
+  %2 = icmp eq i8* %1, null
+  ret i1 %2
+}
+
 declare i8* @llvm.launder.invariant.group.p0i8(i8*)
 declare i8* @llvm.strip.invariant.group.p0i8(i8*)
Index: lib/Analysis/CaptureTracking.cpp
===================================================================
--- lib/Analysis/CaptureTracking.cpp
+++ lib/Analysis/CaptureTracking.cpp
@@ -331,14 +331,30 @@
       AddUses(I);
       break;
     case Instruction::ICmp: {
-      // Don't count comparisons of a no-alias return value against null as
-      // captures. This allows us to ignore comparisons of malloc results
-      // with null, for example.
-      if (ConstantPointerNull *CPN =
-          dyn_cast<ConstantPointerNull>(I->getOperand(1)))
+      if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(1))) {
+        // Don't count comparisons of a no-alias return value against null as
+        // captures. This allows us to ignore comparisons of malloc results
+        // with null, for example.
         if (CPN->getType()->getAddressSpace() == 0)
           if (isNoAliasCall(V->stripPointerCasts()))
             break;
+        auto *O = I->getOperand(0)->stripPointerCasts();
+        // An inbounds GEP can either be a valid pointer (pointing into or
+        // to the end of an allocation), or be null in the default address
+        // space. So for an inbounds GEPs there is no way to let the
+        // pointer escape using clever GEP hacking because doing so would
+        // make the pointer point outside of the allocated object and thus
+        // make the GEP result a poison value.
+        if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
+          if (GEP->isInBounds())
+            break;
+        // Comparing a dereferenceable_or_null argument against null cannot lead
+        // to pointer escapes, because if it is not null it must be a valid
+        // (in-bounds) pointer.
+        if (auto *A = dyn_cast<Argument>(O))
+          if (A->getDereferenceableOrNullBytes())
+            break;
+      }
       // Comparison against value stored in global variable. Given the pointer
       // does not escape, its value cannot be guessed and stored separately in a
       // global variable.


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