I can't seem to wrap my head around the proper fix here,
we should be fine without this requirement, iff we can form this form,
but the naive attempt (https://reviews.llvm.org/D106317) has failed.
So just to unblock the release, put up a restriction.
Fixes https://bugs.llvm.org/show_bug.cgi?id=51125
(cherry picked from commit 909cba969981032c5740774ca84a34b7f76b909b)
When hoisting/moving calls to locations, we strip unknown metadata. Such calls are usually marked `speculatable`, i.e. they are guaranteed to not cause undefined behaviour when run anywhere. So, we should strip attributes that can cause immediate undefined behaviour if those attributes are not valid in the context where the call is moved to.
This patch introduces such an API and uses it in relevant passes. See
updated tests.
Fix for PR50744.
Reviewed By: nikic, jdoerfert, lebedev.ri
Differential Revision: https://reviews.llvm.org/D104641
Nowadays, simplifycfg pass already tail-merges all the ret blocks together
before doing anything, and it should not increase the count of ret's,
so this is dead code.
Proposed alternative to D105338.
This is ugly, but short-term I think it's the best way forward: first,
let's formalize the hacks into a coherent model. Then we can consider
extensions of that model (we could have different flavors of volatile
with different rules).
Differential Revision: https://reviews.llvm.org/D106309
isSafeToSpeculateStore() looks for a preceding store to the same
location to make sure that introducing a new store of the same
value is safe. It currently bails on intervening mayHaveSideEffect()
instructions. However, I believe just checking mayWriteToMemory()
is sufficient there -- we just need to make sure that we know which
value was stored, we don't care if we can unwind in the meantime.
While looking into this, I started having some doubts about the
correctness of the transform with regard to thread safety. While
we don't try to hoist non-simple stores, I believe we also need
to make sure that the preceding store is simple as well. Otherwise
we could introduce a spurious non-atomic write after an atomic write
-- under our memory model this would result in a subsequent undef
atomic read, even if the second write stores the same value as the
first.
Example: https://alive2.llvm.org/ce/z/q_3YAL
Differential Revision: https://reviews.llvm.org/D106742
If the branch isn't `unpredictable`, and it is predicted to *not* branch
to the block we are considering speculatively executing,
then it seems counter-productive to execute the code that is predicted not to be executed.
Reviewed By: spatel
Differential Revision: https://reviews.llvm.org/D106650
Otherwise e.g. the FoldTwoEntryPHINode() has to do a lot of legwork
to re-deduce what is the dominant block (i.e. for which block
is this branch the terminator).
This function is called when some predecessor of an empty return block
ends with a conditional branch, with both successors being empty ret blocks.
Now, because of the way SimplifyCFG works, it might happen to simplify
one of the blocks in a way that makes a conditional branch
into an unconditional one, since it's destinations are now identical,
but it might not have actually simplified said conditional branch
into an unconditional one yet.
So, we have to check that ourselves first,
especially now that SimplifyCFG aggressively tail-merges
all ret and resume blocks.
Even if it was an unconditional branch already,
`SimplifyCFGOpt::simplifyReturn()` doesn't call `FoldReturnIntoUncondBranch()`
by default.
The logical (select) form of and/or will now be a source of problems.
We don't really account for it's inverted form, yet it exists,
and presumably we should treat it just like non-inverted form:
https://alive2.llvm.org/ce/z/BU9AXkhttps://bugs.llvm.org/show_bug.cgi?id=51149 reports a reportedly-serious
perf regression that will hopefully be mitigated by this.
We need to make sure that the value types are the same. Otherwise
we both may not have the necessary dereferenceability implication,
nor can we directly form the desired select pattern.
Without opaque pointers this is enforced implicitly through the
pointer comparison.
`SinkCommonCodeFromPredecessors()` doesn't itself ensure that duplicate PHI nodes aren't created.
I suppose, we could teach it to do that on-the-fly (& account for the already-existing PHI nodes,
& adjust costmodel), the diff will be bigger than this.
The alternative is to schedule a new EarlyCSE pass invocation somewhere later in the pipeline.
Clearly, we don't have any EarlyCSE runs in module optimization passline, so this pattern isn't cleaned up...
That would perhaps better, but it will again have some compile time impact.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D106010
This patch fixes the problem of SimplifyBranchOnICmpChain that occurs
when extra values are Undef or poison.
Suppose the %mode is 51 and the %Cond is poison, and let's look at the
case below.
```
%A = icmp ne i32 %mode, 0
%B = icmp ne i32 %mode, 51
%C = select i1 %A, i1 %B, i1 false
%D = select i1 %C, i1 %Cond, i1 false
br i1 %D, label %T, label %F
=>
br i1 %Cond, label %switch.early.test, label %F
switch.early.test:
switch i32 %mode, label %T [
i32 51, label %F
i32 0, label %F
]
```
incorrectness: https://alive2.llvm.org/ce/z/BWScX
Code before transformation will not raise UB because %C and %D is false,
and it will not use %Cond. But after transformation, %Cond is being used
immediately, and it will raise UB.
This problem can be solved by adding freeze instruction.
correctness: https://alive2.llvm.org/ce/z/x9x4oY
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D104569
This reverts commit 52aeacfbf5ce5f949efe0eae029e56db171ea1f7.
There isn't full agreement on a path forward yet, but there is agreement that
this shouldn't land as-is. See discussion on https://reviews.llvm.org/D105338
Also reverts unreviewed "[clang] Improve `-Wnull-dereference` diag to be more in-line with reality"
This reverts commit f4877c78c0fc98be47b926439bbfe33d5e1d1b6d.
And all the related changes to tests:
This reverts commit 9a0152799f8e4a59e0483728c9f11c8a7805616f.
This reverts commit 3f7c9cc27422f7302cf5a683eeb3978e6cb84270.
This reverts commit 329f8197ef59f9bd23328b52d623ba768b51dbb2.
This reverts commit aa9f58cc2c48ca6cfc853a2467cd775dc7622746.
This reverts commit 2df37d5ddd38091aafbb7d338660e58836f4ac80.
This reverts commit a72a44181264fd83e05be958c2712cbd4560aba7.
This reverts commit 4e413e16216d0c94ada2171f3c59e0a85f4fa4b6,
which landed almost 10 months ago under premise that the original behavior
didn't match reality and was breaking users, even though it was correct as per
the LangRef. But the LangRef change still hasn't appeared, which might suggest
that the affected parties aren't really worried about this problem.
Please refer to discussion in:
* https://reviews.llvm.org/D87399 (`Revert "[InstCombine] erase instructions leading up to unreachable"`)
* https://reviews.llvm.org/D53184 (`[LangRef] Clarify semantics of volatile operations.`)
* https://reviews.llvm.org/D87149 (`[InstCombine] erase instructions leading up to unreachable`)
clang has `-Wnull-dereference` which will diagnose the obvious cases
of null dereference, it was adjusted in f4877c78c0fc98be47b926439bbfe33d5e1d1b6d,
but it will only catch the cases where the pointer is a null literal,
it will not catch the cases where an arbitrary store is expected to trap.
Differential Revision: https://reviews.llvm.org/D105338
This replaces the current ad-hoc implementation,
by syncing the code from InstCombine's implementation in `InstCombinerImpl::visitUnreachableInst()`,
with one exception that here in SimplifyCFG we are allowed to remove EH instructions.
Effectively, this now allows SimplifyCFG to remove calls (iff they won't throw and will return),
arithmetic/logic operations, etc.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D105374
Mimics similar change for InstCombine:
ce192ced2b901be67444c481ab5ca0d731e6d982 / D104602
All these uses are in blocks that aren't reachable from function's entry,
and said blocks are removed by SimplifyCFG itself,
so we can't really test this change.
Somewhat related to D105338.
While it is up for discussion whether or not volatile store traps,
so far there has been no complaints that volatile load/cmpxchg/atomicrmw also may trap.
And even if simplifycfg currently concervatively believes that to be the case,
instcombine does not: https://godbolt.org/z/5vhv4K5b8
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D105343
This problem is exposed by D104598, after it tail-merges `ret` in
`@test_inline_constraint_S_label`, the verifier would start complaining
`invalid operand for inline asm constraint 'S'`.
Essentially, taking address of a block is mismodelled in IR.
It should probably be an explicit instruction, a first one in block,
that isn't identical to any other instruction of the same type,
so that it can't be hoisted.
As a follow-up to https://reviews.llvm.org/D104129, I'm cleaning up the danling probe related code in both the compiler and llvm-profgen.
I'm seeing a 5% size win for the pseudo_probe section for SPEC2017 and 10% for Ciner. Certain benchmark such as 602.gcc has a 20% size win. No obvious difference seen on build time for SPEC2017 and Cinder.
Reviewed By: wenlei
Differential Revision: https://reviews.llvm.org/D104477
The problematic code pattern in the test is based on:
https://llvm.org/PR50638
If the IfCond is itself the phi that we are trying to remove,
then the loop around line 2835 can end up with something like:
%cmp = select i1 %cmp, i1 false, i1 true
That can then lead to a use-after-free and assert (although
I'm still not seeing that locally in my release + asserts build).
I think this can only happen with unreachable code.
Differential Revision: https://reviews.llvm.org/D104063
We are deleting `phi` nodes within the for loop, so this makes sure we
increment the iterator before we delete the instruction pointed by the
iterator.
This started to break in
a0be081646.
Reviewed By: dschuff, lebedev.ri
Differential Revision: https://reviews.llvm.org/D103181
This change tries to fix a place missing `moveAndDanglePseudoProbes `. In FoldValueComparisonIntoPredecessors, it folds the BB into predecessors and then marked the BB unreachable. However, the original logic from the BB is still alive, deleting the probe will mislead the SampleLoader mark it as zero count sample.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D102721
This required some changes to, instead of eagerly making PHI's
in the UnwindDest valid as-if the BB is already not a predecessor,
to be valid while BB is still a predecessor.
Ignore ephemeral values (only feeding llvm.assume intrinsics) when
computing the instruction count to decide if a block is small enough for
threading. This is similar to the handling of these values in the
InlineCost computation. These instructions will eventually be removed
and shouldn't count against code size (similar to the existing ignoring
of phis).
Without this change, when enabling -fwhole-program-vtables, which causes
type test / assume sequences to be inserted by clang, we can get
different threading decisions. In particular, when building with
instrumentation FDO it can affect the optimizations decisions before FDO
matching, leading to some mismatches.
Differential Revision: https://reviews.llvm.org/D101494
We need to use a logical or instead of a bitwise or to preserve
poison behavior. Poison from the second condition should not
propagate if the first condition is true.
We were already handling this correctly in FoldBranchToCommonDest(),
but not in this fold. (There are still other folds with this issue.)
When passingValueIsAlwaysUndefined scans for an instruction between an
inst with a null or undef argument and its first use, it was checking
for instructions that may have side effects, which is a superset of the
instructions it intended to find (as per the comments, control flow
changing instructions that would prevent reaching the uses). Switch
to using isGuaranteedToTransferExecutionToSuccessor() instead.
Without this change, when enabling -fwhole-program-vtables, which causes
assumes to be inserted by clang, we can get different simplification
decisions. In particular, when building with instrumentation FDO it can
affect the optimizations decisions before FDO matching, leading to some
mismatches.
I had to modify d83507-knowledge-retention-bug.ll since this fix enables
more aggressive optimization of that code such that it no longer tested
the original bug it was meant to test. I removed the undef which still
provokes the original failure (confirmed by temporarily reverting the
fix) and also changed it to just invoke the passes of interest to narrow
the testing.
Similarly I needed to adjust code for UnreachableEliminate.ll to avoid
an undef which was causing the function body to get optimized away with
this fix.
Differential Revision: https://reviews.llvm.org/D101507
The profitability check is: we don't want to create more than a single PHI
per instruction sunk. We need to create the PHI unless we'll sink
all of it's would-be incoming values.
But there is a caveat there.
This profitability check doesn't converge on the first iteration!
If we first decide that we want to sink 10 instructions,
but then determine that 5'th one is unprofitable to sink,
that may result in us not sinking some instructions that
resulted in determining that some other instruction
we've determined to be profitable to sink becoming unprofitable.
So we need to iterate until we converge, as in determine
that all leftover instructions are profitable to sink.
But, the direct approach of just re-iterating seems dumb,
because in the worst case we'd find that the last instruction
is unprofitable, which would result in revisiting instructions
many many times.
Instead, i think we can get away with just two passes - forward and backward.
However then it isn't obvious what is the most performant way to update
InstructionsToSink.
While we have a known profitability issue for sinking in presence of
non-unconditional predecessors, there isn't any known issues
for having multiple such non-unconditional predecessors,
so said restriction appears to be artificial. Lift it.
