[PATCH] D66558: [Attributor] Deal with shrinking dereferenceability in a loop

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert created this revision.
jdoerfert added reviewers: uenoku, sstefan1.
Herald added subscribers: bollu, hiraditya.
Herald added a project: LLVM.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D66558

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


Index: llvm/test/Transforms/FunctionAttrs/dereferenceable.ll
===================================================================
--- llvm/test/Transforms/FunctionAttrs/dereferenceable.ll
+++ llvm/test/Transforms/FunctionAttrs/dereferenceable.ll
@@ -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 @@
 
 ; 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
 
@@ -80,3 +81,33 @@
 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
+}
Index: llvm/lib/Transforms/IPO/Attributor.cpp
===================================================================
--- llvm/lib/Transforms/IPO/Attributor.cpp
+++ llvm/lib/Transforms/IPO/Attributor.cpp
@@ -2159,16 +2159,27 @@
       // 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();


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