[llvm-dev] Path condition propagation
Carlos Liam via llvm-dev
llvm-dev at lists.llvm.org
Wed Jul 6 12:09:25 PDT 2016
I have this patch to GVN.cpp, but it doesn't seem to be working; any ideas?
diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index a963b2f..97d7b4b 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -2036,6 +2036,22 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root,
if (RootDominatesEnd)
addToLeaderTable(Num, NotVal, Root.getEnd());
+ // If "A > B" or "A < B", then propagate "(A == B) == false".
+ if (ICmpInst *ICmp = dyn_cast<ICmpInst>(Cmp)) {
+ if (ICmp->isRelational() &&
+ ((isKnownTrue && Cmp->isFalseWhenEqual()) ||
+ (isKnownFalse && Cmp->isTrueWhenEqual()))) {
+ Worklist.push_back(
+ std::make_pair(CmpInst::Create(
+ Cmp->getOpcode(),
+ CmpInst::Predicate::ICMP_EQ,
+ A,
+ B
+ ), ConstantInt::getFalse(Cmp->getContext()))
+ );
+ }
+ }
+
continue;
}
}
- CL
> On Jul 3, 2016, at 8:40 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
> Sure
>
> On Mon, Jul 4, 2016, 9:40 AM Carlos Liam <carlos at aarzee.me <mailto:carlos at aarzee.me>> wrote:
> It looks like there's already something similar in PropagateEquality which eg X >= Y == true and replaces X < Y == false, which is somewhat similar - could I base an addition off of that?
>
>
> - CL
>
> On Jul 3, 2016, at 7:13 PM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>
>> It's going to be really hard to do something sane in the current infrastructure.
>> Its possible, but it would also be slow. You would have to go looking at uses of variables compared in predicates in PropagateEquality and if the uses appear in a comparison that is dominated by the true or false edge of the existing predicate, see if it tells you something about the dominated one.
>>
>>
>> On Mon, Jul 4, 2016, 8:23 AM Carlos Liam <carlos at aarzee.me <mailto:carlos at aarzee.me>> wrote:
>> That seems ominous; should I not bother?
>>
>> - CL
>>
>> On Jul 3, 2016, at 5:58 PM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>>
>>> PropagateEquality in gvn.cpp
>>>
>>> However, if you are going to do it, remember the goal is to make the code simpler and easier, not just pile on to the current mess to catch more cases :)
>>>
>>> On Mon, Jul 4, 2016, 7:51 AM Carlos Liam <carlos at aarzee.me <mailto:carlos at aarzee.me>> wrote:
>>> Where would I look to change the equality propagation?
>>>
>>>
>>> - CL
>>>
>>> On Jun 30, 2016, at 11:45 PM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>>>
>>>> The current gvn equality propagation is not powerful enough to get this because it doesn't try to infer values in predicates based on other predicates, so it never realizes a>b -> a !=b in a useful way.
>>>>
>>>> It otherwise would get this
>>>>
>>>> On Thu, Jun 30, 2016, 7:41 PM Sean Silva <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
>>>> On Thu, Jun 30, 2016 at 6:45 PM, Daniel Berlin via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>>
>>>>
>>>> On Thu, Jun 30, 2016 at 6:09 PM, Carlos Liam via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>> Hi all,
>>>>
>>>> Consider this C code:
>>>>
>>>> #include <stdbool.h>
>>>>
>>>> bool func(int n1, int n2, bool b) {
>>>> bool greater = n1 > n2;
>>>> if (greater && b) {
>>>> if (n1 == n2) {
>>>> return false; // unreachable
>>>> }
>>>> }
>>>> return true;
>>>> }
>>>>
>>>> The line marked unreachable cannot be reached, however currently LLVM does not optimize it out
>>>> ?????
>>>> Yes it does.
>>>>
>>>> It seems like we get this almost by accident though. I find that I need `-mem2reg -instcombine -simplifycfg -instcombine` (on clang -O0 IR; the `-mem2reg -instcombine` are just cleanup) and essentially it boils down to simplifycfg merging everything into a single branch-free expression and then instcombine algebraically merging the comparisons.
>>>>
>>>> A small modification defeats LLVM's optimizer:
>>>>
>>>> bool func(int n1, int n2, bool b) {
>>>> bool greater = n1 > n2;
>>>> if (greater && b) {
>>>> foo();
>>>>
>>>> if (n1 == n2) {
>>>> return false; // unreachable
>>>> }
>>>> }
>>>> return true;
>>>> }
>>>>
>>>> In this case, simplifycfg doesn't go wild merging everything into a single branch-free expression and so we don't get it.
>>>>
>>>>
>>>> CorrelatedValuePropagation doesn't get this because its processCmp is quite weak (it bails out if one operand isn't a constant). JumpThreading is the only other pass that uses LazyValueInfo and it can't fold this since it can't thread a jump around the side-effecting `foo()` call.
>>>>
>>>> I'm not familiar with GVN but it doesn't seem to help for this modified test case either.
>>>>
>>>> Carlos, in answer to your original question, you may want to see if you can make LLVM get this case by modifying processCmp in lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
>>>>
>>>> -- Sean Silva
>>>>
>>>> [dannyb at dannyb-macbookpro3 18:39:18] ~/sources/llvm (git-svn)-[newgvn-predicates]- :( $ clang -c -emit-llvm ~/greater.c -O1
>>>> [dannyb at dannyb-macbookpro3 18:39:22] ~/sources/llvm (git-svn)-[newgvn-predicates]- :) $ debug-build/bin/llvm-dis greater.bc
>>>> [dannyb at dannyb-macbookpro3 18:39:24] ~/sources/llvm (git-svn)-[newgvn-predicates]- :) $ cat greater.ll
>>>> ; Function Attrs: norecurse nounwind readnone ssp uwtable
>>>> define zeroext i1 @func(i32, i32, i1 zeroext) #0 {
>>>> ret i1 true
>>>> }
>>>>
>>>>
>>>> opt -simplifycfg -instcombine does the same thing to it if you use -O0 with clang
>>>>
>>>> I believe this is because LLVM does not recognize that meeting path conditions like, for example, X && Y logically means that X is true and Y is true.
>>>>
>>>>
>>>> Yes it does. See both GVN's propagateequality and correlatedvaluepropagation, among other things :)
>>>>
>>>> In this case, simplifycfg +instcombine will do it
>>>>
>>>> The new predicate support i'm building for GVN will also do it.
>>>>
>>>>
>>>> I'm interested in creating a patch to remedy this; is there a file or function I should look at?
>>>>
>>>> Thanks in advance.
>>>>
>>>> - CL
>>>> _______________________________________________
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>>>>
>>>>
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>>>>
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