[Instcombine] Disable memcpy of alloca bypass for instruction sources

This transformation is fundamentally broken when it comes to dominance,
it just happened to work when the source of the memcpy can be moved into
the place of the alloca. The bug shows up a lot more often since
077bff39d46364035a5dcfa32fc69910ad0975d0 allows the source to be a
switch.

It would be possible to check dominance of the source and all its
operands, but that seems very heavy for instcombine.

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 3ca5495..dd4c15e 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -407,10 +407,9 @@ Instruction *InstCombinerImpl::visitAllocaInst(AllocaInst &AI) {
       TheSrc, AllocaAlign, DL, &AI, &AC, &DT);
     if (AllocaAlign <= SourceAlign &&
         isDereferenceableForAllocaSize(TheSrc, &AI, DL) &&
-        (!isa<Instruction>(TheSrc) || all_of(AI.users(), [&](const User *U) {
-          return DT.dominates(cast<Instruction>(TheSrc), cast<Instruction>(U));
-        }))) {
-      // FIXME: can the dominance restriction be relaxed by sinking instrns?
+        !isa<Instruction>(TheSrc)) {
+      // FIXME: Can we sink instructions without violating dominance when TheSrc
+      // is an instruction instead of a constant or argument?
       LLVM_DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
       LLVM_DEBUG(dbgs() << "  memcpy = " << *Copy << '\n');
       unsigned SrcAddrSpace = TheSrc->getType()->getPointerAddressSpace();

diff --git a/llvm/test/Transforms/InstCombine/tmp-alloca-bypass.ll b/llvm/test/Transforms/InstCombine/tmp-alloca-bypass.ll
index 9a87e8a..7fb531d 100644 (file)
--- a/llvm/test/Transforms/InstCombine/tmp-alloca-bypass.ll
+++ b/llvm/test/Transforms/InstCombine/tmp-alloca-bypass.ll
@@ -31,5 +31,29 @@ define void @test(i8*%out) {
   ret void
 }
 
+define void @test2() {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[I:%.*]] = alloca [[T0:%.*]], align 8
+; CHECK-NEXT:    [[I1:%.*]] = call i32 @func(%t0* undef)
+; CHECK-NEXT:    [[I2:%.*]] = icmp eq i32 [[I1]], 2503
+; CHECK-NEXT:    [[I3:%.*]] = select i1 [[I2]], i8* bitcast (%t0* @g0 to i8*), i8* bitcast (%t0* @g1 to i8*)
+; CHECK-NEXT:    [[I4:%.*]] = bitcast %t0* [[I]] to i8*
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* noundef nonnull align 8 dereferenceable(16) [[I4]], i8* noundef nonnull align 8 dereferenceable(16) [[I3]], i64 16, i1 false)
+; CHECK-NEXT:    [[I5:%.*]] = call i32 @func(%t0* nonnull byval([[T0]]) [[I]])
+; CHECK-NEXT:    unreachable
+;
+bb:
+  %i = alloca %t0, align 8
+  %i1 = call i32 @func(%t0* undef)
+  %i2 = icmp eq i32 %i1, 2503
+  %i3 = select i1 %i2, i8* bitcast (%t0* @g0 to i8*), i8* bitcast (%t0* @g1 to i8*)
+  %i4 = bitcast %t0* %i to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* noundef nonnull align 8 dereferenceable(16) %i4, i8* noundef nonnull align 8 dereferenceable(16) %i3, i64 16, i1 false)
+  %i5 = call i32 @func(%t0* nonnull byval(%t0) %i)
+  unreachable
+}
+
+declare i32 @func(%t0*)
 declare i1 @get_cond()
 declare void @llvm.memcpy.p0i8.p0i8.i64(i8*, i8*, i64, i1)