[llvm] r369784 - [Attributor] Deal with shrinking dereferenceability in a loop

[Johannes Doerfert via llvm-commits]

Author: jdoerfert
Date: Fri Aug 23 10:29:23 2019
New Revision: 369784

URL: http://llvm.org/viewvc/llvm-project?rev=369784&view=rev
Log:
[Attributor] Deal with shrinking dereferenceability in a loop

Summary:
If we have a loop in which the dereferenceability of a pointer decreases
we did slowly decrease it iteration by iteration, leading to a timeout.
With this patch we detect such circular reasoning and indicate a
fixpoint early.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66558

Modified:
    llvm/trunk/lib/Transforms/IPO/Attributor.cpp
    llvm/trunk/test/Transforms/FunctionAttrs/dereferenceable.ll

Modified: llvm/trunk/lib/Transforms/IPO/Attributor.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/Attributor.cpp?rev=369784&r1=369783&r2=369784&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/Attributor.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/Attributor.cpp Fri Aug 23 10:29:23 2019
@@ -1988,16 +1988,27 @@ struct AADereferenceableFloating : AADer
       // For now we do not try to "increase" dereferenceability due to negative
       // indices as we first have to come up with code to deal with loops and
       // for overflows of the dereferenceable bytes.
-      if (Offset.getSExtValue() < 0)
+      int64_t OffsetSExt = Offset.getSExtValue();
+      if (OffsetSExt < 0)
         Offset = 0;
 
       T.takeAssumedDerefBytesMinimum(
-          std::max(int64_t(0), DerefBytes - Offset.getSExtValue()));
+          std::max(int64_t(0), DerefBytes - OffsetSExt));
 
-      if (!Stripped && this == &AA) {
-        T.takeKnownDerefBytesMaximum(
-            std::max(int64_t(0), DerefBytes - Offset.getSExtValue()));
-        T.indicatePessimisticFixpoint();
+      if (this == &AA) {
+        if (!Stripped) {
+          // If nothing was stripped IR information is all we got.
+          T.takeKnownDerefBytesMaximum(
+              std::max(int64_t(0), DerefBytes - OffsetSExt));
+          T.indicatePessimisticFixpoint();
+        } else if (OffsetSExt > 0) {
+          // If something was stripped but there is circular reasoning we look
+          // for the offset. If it is positive we basically decrease the
+          // dereferenceable bytes in a circluar loop now, which will simply
+          // drive them down to the known value in a very slow way which we
+          // can accelerate.
+          T.indicatePessimisticFixpoint();
+        }
       }
 
       return T.isValidState();

Modified: llvm/trunk/test/Transforms/FunctionAttrs/dereferenceable.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionAttrs/dereferenceable.ll?rev=369784&r1=369783&r2=369784&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/FunctionAttrs/dereferenceable.ll (original)
+++ llvm/trunk/test/Transforms/FunctionAttrs/dereferenceable.ll Fri Aug 23 10:29:23 2019
@@ -1,6 +1,8 @@
 ; RUN: opt -attributor --attributor-disable=false -attributor-max-iterations=4 -S < %s | FileCheck %s --check-prefixes=ATTRIBUTOR
 
 
+declare void @deref_phi_user(i32* %a);
+
 ; TEST 1
 ; take mininimum of return values
 ;
@@ -52,8 +54,7 @@ define dereferenceable(4) i32* @test4(i3
 
 ; TEST 5
 ; loop in which dereferenceabily "grows"
-declare void @deref_phi_user(i32* %a);
-define void @deref_phi(i32* dereferenceable(4000) %a) {
+define void @deref_phi_growing(i32* dereferenceable(4000) %a) {
 entry:
   br label %for.cond
 
@@ -77,6 +78,36 @@ for.inc:
   %inc = add nuw nsw i32 %i.0, 1
   br label %for.cond
 
+for.end:                                          ; preds = %for.cond.cleanup
+  ret void
+}
+
+; TEST 6
+; loop in which dereferenceabily "shrinks"
+define void @deref_phi_shrinking(i32* dereferenceable(4000) %a) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %a.addr.0 = phi i32* [ %a, %entry ], [ %incdec.ptr, %for.inc ]
+; CHECK: call void @deref_phi_user(i32* %a.addr.0)
+  call void @deref_phi_user(i32* %a.addr.0)
+  %tmp = load i32, i32* %a.addr.0, align 4
+  %cmp = icmp slt i32 %i.0, %tmp
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  br label %for.end
+
+for.body:                                         ; preds = %for.cond
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %incdec.ptr = getelementptr inbounds i32, i32* %a.addr.0, i64 1
+  %inc = add nuw nsw i32 %i.0, 1
+  br label %for.cond
+
 for.end:                                          ; preds = %for.cond.cleanup
   ret void
 }