<div dir="ltr">btw, we're seeing something that looks like miscompiles with this patch (& after the fix in r369795). The code looks pretty well defined (passes asan/msan/etc.).<div><br></div><div>I'm trying to reduce it into something that actually triggers the bug, but it vaguely looks like:</div><div>struct Foo {</div><div> bool IsValid() const { return x; }</div><div> Foo(char *y): x(y) {}</div><div> char *x;</div><div>}</div><div><br></div><div>Foo get() {</div><div> return some_condition ? Foo(<valid pointer>) : Foo(nullptr);</div><div>}</div><div><br></div><div>void doSomething() {</div><div> Foo foo = get();</div><div> if (foo.IsValid()) { .. }</div><div>}</div><div><br></div><div>What I'm observing is get() go down the branch of some_condition being true and returning Foo(<valid pointer>), but foo.IsValid() is returning false.</div><div><br>Given that context, is there anything you might see is a subtle bug with this patch?</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Aug 23, 2019 at 10:57 AM Philip Reames via llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org">llvm-commits@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Author: reames<br>
Date: Fri Aug 23 10:58:58 2019<br>
New Revision: 369789<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=369789&view=rev" rel="noreferrer" target="_blank">http://llvm.org/viewvc/llvm-project?rev=369789&view=rev</a><br>
Log:<br>
[InstCombine] icmp eq/ne (gep inbounds P, Idx..), null -> icmp eq/ne P, null<br>
<br>
This generalizes the isGEPKnownNonNull rule from ValueTracking to apply when we do not know if the base is non-null, and thus need to replace one condition with another.<br>
<br>
The core notion is that since an inbounds GEP can only form null if the base pointer is null and the offset is zero. However, if the offset is non-zero, the the "inbounds" marker makes the result poison. Thus, we're free to ignore the case where the offset is non-zero. Similarly, there's no case under which a non-null base can result in a null result without generating poison.<br>
<br>
Differential Revision: <a href="https://reviews.llvm.org/D66608" rel="noreferrer" target="_blank">https://reviews.llvm.org/D66608</a><br>
<br>
<br>
Added:<br>
llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll<br>
Modified:<br>
llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp<br>
<br>
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=369789&r1=369788&r2=369789&view=diff" rel="noreferrer" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=369789&r1=369788&r2=369789&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)<br>
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Fri Aug 23 10:58:58 2019<br>
@@ -894,6 +894,27 @@ Instruction *InstCombiner::foldGEPICmp(G<br>
Offset = EmitGEPOffset(GEPLHS);<br>
return new ICmpInst(ICmpInst::getSignedPredicate(Cond), Offset,<br>
Constant::getNullValue(Offset->getType()));<br>
+ } else if (GEPLHS->isInBounds() && ICmpInst::isEquality(Cond) &&<br>
+ GEPLHS->getType()->isPointerTy() && // TODO: extend to vector geps<br>
+ isa<Constant>(RHS) && cast<Constant>(RHS)->isNullValue() &&<br>
+ !NullPointerIsDefined(I.getFunction(),<br>
+ RHS->getType()->getPointerAddressSpace())) {<br>
+ // For most address spaces, an allocation can't be placed at null, but null<br>
+ // itself is treated as a 0 size allocation in the in bounds rules. Thus,<br>
+ // the only valid inbounds address derived from null, is null itself.<br>
+ // Thus, we have four cases to consider:<br>
+ // 1) Base == nullptr, Offset == 0 -> inbounds, null<br>
+ // 2) Base == nullptr, Offset != 0 -> poison as the result is out of bounds<br>
+ // 3) Base != nullptr, Offset == (-base) -> poison (crossing allocations)<br>
+ // 4) Base != nullptr, Offset != (-base) -> nonnull (and possibly poison)<br>
+ //<br>
+ // (Note if we're indexing a type of size 0, that simply collapses into one<br>
+ // of the buckets above.)<br>
+ //<br>
+ // In general, we're allowed to make values less poison (i.e. remove<br>
+ // sources of full UB), so in this case, we just select between the two<br>
+ // non-poison cases (1 and 4 above).<br>
+ return new ICmpInst(Cond, GEPLHS->getPointerOperand(), RHS);<br>
} else if (GEPOperator *GEPRHS = dyn_cast<GEPOperator>(RHS)) {<br>
// If the base pointers are different, but the indices are the same, just<br>
// compare the base pointer.<br>
<br>
Added: llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll?rev=369789&view=auto" rel="noreferrer" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll?rev=369789&view=auto</a><br>
==============================================================================<br>
--- llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll (added)<br>
+++ llvm/trunk/test/Transforms/InstCombine/gep-inbounds-null.ll Fri Aug 23 10:58:58 2019<br>
@@ -0,0 +1,184 @@<br>
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py<br>
+; RUN: opt -S < %s -instcombine | FileCheck %s<br>
+<br>
+;; Start by showing the results of constant folding (which doesn't use<br>
+;; the poison implied by gep for the nonnull cases).<br>
+<br>
+define i1 @test_ne_constants_null() {<br>
+; CHECK-LABEL: @test_ne_constants_null(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 false<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* null, i64 0<br>
+ %cnd = icmp ne i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @test_ne_constants_nonnull() {<br>
+; CHECK-LABEL: @test_ne_constants_nonnull(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 true<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* null, i64 1<br>
+ %cnd = icmp ne i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @test_eq_constants_null() {<br>
+; CHECK-LABEL: @test_eq_constants_null(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 true<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* null, i64 0<br>
+ %cnd = icmp eq i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @test_eq_constants_nonnull() {<br>
+; CHECK-LABEL: @test_eq_constants_nonnull(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 false<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* null, i64 1<br>
+ %cnd = icmp eq i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+;; Then show the results for non-constants. These use the inbounds provided<br>
+;; UB fact to ignore the possible overflow cases.<br>
+<br>
+define i1 @test_ne(i8* %base, i64 %idx) {<br>
+; CHECK-LABEL: @test_ne(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp ne i8* [[BASE:%.*]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* %base, i64 %idx<br>
+ %cnd = icmp ne i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @test_eq(i8* %base, i64 %idx) {<br>
+; CHECK-LABEL: @test_eq(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp eq i8* [[BASE:%.*]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* %base, i64 %idx<br>
+ %cnd = icmp eq i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+;; TODO: vectors not yet handled<br>
+define <2 x i1> @test_vector_base(<2 x i8*> %base, i64 %idx) {<br>
+; CHECK-LABEL: @test_vector_base(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, <2 x i8*> [[BASE:%.*]], i64 [[IDX:%.*]]<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp eq <2 x i8*> [[GEP]], zeroinitializer<br>
+; CHECK-NEXT: ret <2 x i1> [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, <2 x i8*> %base, i64 %idx<br>
+ %cnd = icmp eq <2 x i8*> %gep, zeroinitializer<br>
+ ret <2 x i1> %cnd<br>
+}<br>
+<br>
+define <2 x i1> @test_vector_index(i8* %base, <2 x i64> %idx) {<br>
+; CHECK-LABEL: @test_vector_index(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, i8* [[BASE:%.*]], <2 x i64> [[IDX:%.*]]<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp eq <2 x i8*> [[GEP]], zeroinitializer<br>
+; CHECK-NEXT: ret <2 x i1> [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* %base, <2 x i64> %idx<br>
+ %cnd = icmp eq <2 x i8*> %gep, zeroinitializer<br>
+ ret <2 x i1> %cnd<br>
+}<br>
+<br>
+;; These two show instsimplify's reasoning getting to the non-zero offsets<br>
+;; before instcombine does.<br>
+<br>
+define i1 @test_eq_pos_idx(i8* %base) {<br>
+; CHECK-LABEL: @test_eq_pos_idx(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 false<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* %base, i64 1<br>
+ %cnd = icmp eq i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @test_eq_neg_idx(i8* %base) {<br>
+; CHECK-LABEL: @test_eq_neg_idx(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: ret i1 false<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8* %base, i64 -1<br>
+ %cnd = icmp eq i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+;; Show an example with a zero sized type since that's<br>
+;; a cornercase which keeps getting mentioned. The GEP<br>
+;; produces %base regardless of the value of the index<br>
+;; expression.<br>
+define i1 @test_size0({}* %base, i64 %idx) {<br>
+; CHECK-LABEL: @test_size0(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp ne {}* [[BASE:%.*]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds {}, {}* %base, i64 %idx<br>
+ %cnd = icmp ne {}* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+define i1 @test_size0_nonzero_offset({}* %base) {<br>
+; CHECK-LABEL: @test_size0_nonzero_offset(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp ne {}* [[BASE:%.*]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds {}, {}* %base, i64 15<br>
+ %cnd = icmp ne {}* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+<br>
+;; Finally, some negative tests for sanity checking.<br>
+<br>
+define i1 @neq_noinbounds(i8* %base, i64 %idx) {<br>
+; CHECK-LABEL: @neq_noinbounds(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, i8* [[BASE:%.*]], i64 [[IDX:%.*]]<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp ne i8* [[GEP]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr i8, i8* %base, i64 %idx<br>
+ %cnd = icmp ne i8* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
+<br>
+define i1 @neg_objectatnull(i8 addrspace(2)* %base, i64 %idx) {<br>
+; CHECK-LABEL: @neg_objectatnull(<br>
+; CHECK-NEXT: entry:<br>
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, i8 addrspace(2)* [[BASE:%.*]], i64 [[IDX:%.*]]<br>
+; CHECK-NEXT: [[CND:%.*]] = icmp eq i8 addrspace(2)* [[GEP]], null<br>
+; CHECK-NEXT: ret i1 [[CND]]<br>
+;<br>
+entry:<br>
+ %gep = getelementptr inbounds i8, i8 addrspace(2)* %base, i64 %idx<br>
+ %cnd = icmp eq i8 addrspace(2)* %gep, null<br>
+ ret i1 %cnd<br>
+}<br>
<br>
<br>
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</blockquote></div>