[PATCH] D40802: [MemDep] Don't use cached results from a previous larger query

[Dan Gohman via Phabricator via llvm-commits]

sunfish created this revision.
Herald added a subscriber: aheejin.

When a memdep query finds a cached result from a previous query that had a larger size, don't reuse the cached results, because aliasing queries with a greater size aren't always more conservative. For example, BasicAA knows that accesses larger than the size of underlying objects don't alias those objects.

This could increase compile time in the case of many queries to the same pointer with successively smaller sizes, though this is uncommon.

Fixes PR35519.


Repository:
  rL LLVM

https://reviews.llvm.org/D40802

Files:
  lib/Analysis/MemoryDependenceAnalysis.cpp
  test/Transforms/MemCpyOpt/mixed-sizes.ll


Index: lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- lib/Analysis/MemoryDependenceAnalysis.cpp
+++ lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -1126,21 +1126,15 @@
   // If we already have a cache entry for this CacheKey, we may need to do some
   // work to reconcile the cache entry and the current query.
   if (!Pair.second) {
-    if (CacheInfo->Size < Loc.Size) {
-      // The query's Size is greater than the cached one. Throw out the
-      // cached data and proceed with the query at the greater size.
+    if (CacheInfo->Size != Loc.Size) {
+      // The query's Size differs from the cached one. Throw out the
+      // cached data and proceed with the query at the new size.
       CacheInfo->Pair = BBSkipFirstBlockPair();
       CacheInfo->Size = Loc.Size;
       for (auto &Entry : CacheInfo->NonLocalDeps)
         if (Instruction *Inst = Entry.getResult().getInst())
           RemoveFromReverseMap(ReverseNonLocalPtrDeps, Inst, CacheKey);
       CacheInfo->NonLocalDeps.clear();
-    } else if (CacheInfo->Size > Loc.Size) {
-      // This query's Size is less than the cached one. Conservatively restart
-      // the query using the greater size.
-      return getNonLocalPointerDepFromBB(
-          QueryInst, Pointer, Loc.getWithNewSize(CacheInfo->Size), isLoad,
-          StartBB, Result, Visited, SkipFirstBlock);
     }
 
     // If the query's AATags are inconsistent with the cached one,
Index: test/Transforms/MemCpyOpt/mixed-sizes.ll
===================================================================
--- test/Transforms/MemCpyOpt/mixed-sizes.ll
+++ test/Transforms/MemCpyOpt/mixed-sizes.ll
@@ -0,0 +1,36 @@
+; RUN: opt < %s -memcpyopt -S | FileCheck %s
+; Handle memcpy-memcpy dependencies of differing sizes correctly.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; Don't delete the second memcpy, even though there's an earlier
+; memcpy with a larger size from the same address.
+
+; CHECK-LABEL: @foo
+define i32 @foo(i1 %z) {
+entry:
+  %a = alloca [10 x i32]
+  %s = alloca [10 x i32]
+  %0 = bitcast [10 x i32]* %a to i8*
+  %1 = bitcast [10 x i32]* %s to i8*
+  call void @llvm.memset.p0i8.i64(i8* nonnull %1, i8 0, i64 40, i32 16, i1 false)
+  %arrayidx = getelementptr inbounds [10 x i32], [10 x i32]* %a, i64 0, i64 0
+  store i32 1, i32* %arrayidx
+  %scevgep = getelementptr [10 x i32], [10 x i32]* %s, i64 0, i64 1
+  %scevgep7 = bitcast i32* %scevgep to i8*
+  br i1 %z, label %for.body3.lr.ph, label %for.inc7.1
+
+for.body3.lr.ph:                                  ; preds = %entry
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %scevgep7, i64 17179869180, i32 4, i1 false)
+  br label %for.inc7.1
+
+for.inc7.1:
+; CHECK: for.inc7.1:
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %scevgep7, i64 4, i32 4, i1 false)
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %scevgep7, i64 4, i32 4, i1 false)
+  %2 = load i32, i32* %arrayidx
+  ret i32 %2
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8*, i8*, i64, i32, i1)
+declare void @llvm.memset.p0i8.i64(i8*, i8, i64, i32, i1)


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