[PATCH] D120303: [SCEV] Do not erase LoopUsers. PR53969

Mon Feb 21 22:59:29 PST 2022

mkazantsev created this revision.
mkazantsev added reviewers: reames, lebedev.ri, nikic, fhahn.
Herald added subscribers: javed.absar, hiraditya.
mkazantsev requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

This patch fixes a logical error in how we work with `LoopUsers` map.
It maps a loop onto a set of AddRecs that depend on it. The Addrecs
are added to this map only once when they are created as `UniqueSCEVs`.

The only purpose of this map is to make sure that, whenever we forget
a loop, all (directly or indirectly) dependent SCEVs get forgotten too.

Current code erases SCEVs from dependent set of a given loop whenever
we forget this loop. This is not a correct behavior due to the following scenario:

1. We have a loop `L` and an AddRec `AR` that depends on it;
2. We modify something in the loop, but don't destroy it. We still call forgetLoop on it;
3. `AR` is no longer dependent on `L` according to `LoopUsers`. It is erased from ValueExprMap` and `ExprValue map, but still exists in UniqueSCEVs;
4. We can later request the very same AddRec for the very same loop again, and get existing SCEV `AR`.
5. Now, `AR` exists and is used again, but its notion that it depends on `L` is lost;
6. Then we decide to delete `L`. `AR` will not be forgotten because we have lost it;
7. Just you wait when you run into a dangling pointer problem, or any other kind of problem because an active SCEV is now referecing a non-existent loop.

The solution to this is to stop erasing values from `LoopUsers`. Yes, we will maybe forget something
that is already not used, but it's cheap. The only real problem we can run into is following: if we
delete a loop, and then create a new loop by the same address, this may cause cache confusion.
However, this still should be safe because the *only* purpose of LoopUsers is invalidation, and
invalidation never reads loops from AddRecs.

This fixes a functional bug and potentially may have negative compile time impact on methods with
huge or numerous loops.


https://reviews.llvm.org/D120303

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Transforms/LoopDeletion/pr53969.ll


Index: llvm/test/Transforms/LoopDeletion/pr53969.ll
===================================================================

--- llvm/test/Transforms/LoopDeletion/pr53969.ll
+++ llvm/test/Transforms/LoopDeletion/pr53969.ll
@@ -1,13 +1,69 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -passes="loop(indvars,loop-deletion)" -S  < %s | FileCheck %s
-; XFAIL: *
-; REQUIRES: asserts
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128-ni:1-p2:32:8:8:32-ni:2"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Make sure we don't crash.
 define void @test() {
-; CHECK-LABEL: test
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[BB1:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi i32 [ 11, [[BB:%.*]] ]
+; CHECK-NEXT:    [[TMP3:%.*]] = add nsw i32 112, -1
+; CHECK-NEXT:    [[TMP4:%.*]] = add nuw nsw i32 [[TMP2]], 1
+; CHECK-NEXT:    [[TMP5:%.*]] = mul i32 [[TMP3]], [[TMP3]]
+; CHECK-NEXT:    [[TMP6:%.*]] = mul nsw i32 [[TMP2]], -6
+; CHECK-NEXT:    [[TMP7:%.*]] = mul nsw i32 [[TMP6]], [[TMP5]]
+; CHECK-NEXT:    [[TMP8:%.*]] = add nuw nsw i32 [[TMP7]], [[TMP2]]
+; CHECK-NEXT:    [[TMP9:%.*]] = and i32 undef, 1
+; CHECK-NEXT:    [[TMP10:%.*]] = icmp eq i32 [[TMP9]], 0
+; CHECK-NEXT:    br i1 [[TMP10]], label [[BB33_LOOPEXIT1:%.*]], label [[BB34_PREHEADER:%.*]]
+; CHECK:       bb34.preheader:
+; CHECK-NEXT:    br label [[BB34:%.*]]
+; CHECK:       bb11:
+; CHECK-NEXT:    [[TMP2_LCSSA12:%.*]] = phi i32 [ 11, [[BB34]] ]
+; CHECK-NEXT:    br label [[BB33_LOOPEXIT:%.*]]
+; CHECK:       bb12:
+; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[TMP40:%.*]], 0
+; CHECK-NEXT:    br label [[BB14:%.*]]
+; CHECK:       bb14:
+; CHECK-NEXT:    br i1 true, label [[BB32:%.*]], label [[BB22:%.*]]
+; CHECK:       bb22:
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 4 to i32
+; CHECK-NEXT:    [[TMP23:%.*]] = or i32 [[TMP1]], undef
+; CHECK-NEXT:    [[TMP24:%.*]] = add i32 [[TMP23]], undef
+; CHECK-NEXT:    br i1 false, label [[BB42:%.*]], label [[BB25:%.*]]
+; CHECK:       bb25:
+; CHECK-NEXT:    br label [[BB31:%.*]]
+; CHECK:       bb31:
+; CHECK-NEXT:    unreachable
+; CHECK:       bb32:
+; CHECK-NEXT:    ret void
+; CHECK:       bb33.loopexit:
+; CHECK-NEXT:    [[TMP2_LCSSA9:%.*]] = phi i32 [ [[TMP2_LCSSA12]], [[BB11:%.*]] ]
+; CHECK-NEXT:    br label [[BB33:%.*]]
+; CHECK:       bb33.loopexit1:
+; CHECK-NEXT:    [[TMP2_LCSSA:%.*]] = phi i32 [ [[TMP2]], [[BB1]] ]
+; CHECK-NEXT:    br label [[BB33]]
+; CHECK:       bb33:
+; CHECK-NEXT:    [[TMP210:%.*]] = phi i32 [ [[TMP2_LCSSA]], [[BB33_LOOPEXIT1]] ], [ [[TMP2_LCSSA9]], [[BB33_LOOPEXIT]] ]
+; CHECK-NEXT:    call void @use(i32 [[TMP210]])
+; CHECK-NEXT:    ret void
+; CHECK:       bb34:
+; CHECK-NEXT:    [[TMP36:%.*]] = xor i32 0, [[TMP8]]
+; CHECK-NEXT:    [[TMP38:%.*]] = add i32 [[TMP36]], undef
+; CHECK-NEXT:    [[TMP39:%.*]] = add i32 [[TMP38]], undef
+; CHECK-NEXT:    [[TMP40]] = sext i32 [[TMP39]] to i64
+; CHECK-NEXT:    br i1 false, label [[BB11]], label [[BB12:%.*]]
+; CHECK:       bb42:
+; CHECK-NEXT:    [[TMP24_LCSSA:%.*]] = phi i32 [ [[TMP24]], [[BB22]] ]
+; CHECK-NEXT:    [[TMP18_LCSSA4:%.*]] = phi i64 [ [[TMP0]], [[BB22]] ]
+; CHECK-NEXT:    store atomic i64 [[TMP18_LCSSA4]], i64 addrspace(1)* undef unordered, align 8
+; CHECK-NEXT:    call void @use(i32 [[TMP24_LCSSA]])
+; CHECK-NEXT:    ret void
+;
 bb:
   br label %bb1
 
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8068,7 +8068,6 @@
     if (LoopUsersItr != LoopUsers.end()) {
       ToForget.insert(ToForget.end(), LoopUsersItr->second.begin(),
                 LoopUsersItr->second.end());
-      LoopUsers.erase(LoopUsersItr);
     }
 
     // Drop information about expressions based on loop-header PHIs.


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