This is a partial fix for https://bugs.llvm.org/show_bug.cgi?id=44403.
Folding gep p, q-p to q is only legal if p and q have the same
provenance. This fold should probably be guarded by something like
getUnderlyingObject(p) == getUnderlyingObject(q).
This patch is a partial fix that removes the special handling for
gep p, 0-p, which will fold to a null pointer, which would certainly
not pass an underlying object check (unless p is also null, in which
case this would fold trivially anyway). Folding to a null pointer
is particularly problematic due to the special handling it receives
in many places, making end-to-end miscompiles more likely.
Differential Revision: https://reviews.llvm.org/D93820
Similar to the Arm VCTP intrinsics, if the operands of an
active.lane.mask are both known, the constant lane mask can be
calculated. This can come up after unrolling the loops.
Differential Revision: https://reviews.llvm.org/D94103
Calling null or undef results in immediate undefined behavior.
Return poison instead of undef in this case, similar to what
we do for immediate UB due to division by zero.
Make InstSimplify return poison rather than undef for out-of-bounds
shifts, as specified by LandRef:
> If op2 is (statically or dynamically) equal to or larger than the
> number of bits in op1, this instruction returns a poison value.
Differential Revision: https://reviews.llvm.org/D93998
As the comment already indicates, performing an operation with
nnan/ninf flags on a nan/inf or undef results in poison. Now that
we have a proper poison value, we no longer need to relax it to
undef.
Div/rem by zero is immediate undefined behavior and anything goes.
Currently we fold it to undef, this patch changes it to fold to
poison instead, which is slightly stronger.
Differential Revision: https://reviews.llvm.org/D93995
This is the same change as D93990, but for extractelement rather
than insertelement.
> If idx exceeds the length of val for a fixed-length vector, the
> result is a poison value. For a scalable vector, if the value of
> idx exceeds the runtime length of the vector, the result is a
> poison value.
This is a simple patch that updates InstSimplify to return poison if the index is/can be out-of-bounds
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D93990
This commit copies existing tests at llvm/Transforms containing
'shufflevector X, undef' and replaces them with 'shufflevector X, poison'.
The new copied tests have *-inseltpoison.ll suffix at its file name
(as db7a2f347f did)
See https://reviews.llvm.org/D93793
Test files listed using
grep -R -E "^[^;]*shufflevector <.*> .*, <.*> undef" | cut -d":" -f1 | uniq
Test files copied & updated using
file_org=llvm/test/Transforms/$1
if [[ "$file_org" = *-inseltpoison.ll ]]; then
file=$file_org
else
file=${file_org%.ll}-inseltpoison.ll
if [ ! -f $file ]; then
cp $file_org $file
fi
fi
sed -i -E 's/^([^;]*)shufflevector <(.*)> (.*), <(.*)> undef/\1shufflevector <\2> \3, <\4> poison/g' $file
head -1 $file | grep "Assertions have been autogenerated by utils/update_test_checks.py" -q
if [ "$?" == 1 ]; then
echo "$file : should be manually updated"
# The test is manually updated
exit 1
fi
python3 ./llvm/utils/update_test_checks.py --opt-binary=./build-releaseassert/bin/opt $file
This commit copies existing tests at llvm/Transforms and replaces
'insertelement undef' in those files with 'insertelement poison'.
(see https://reviews.llvm.org/D93586)
Tests listed using this script:
grep -R -E '^[^;]*insertelement <.*> undef,' . | cut -d":" -f1 | uniq |
wc -l
Tests updated:
file_org=llvm/test/Transforms/$1
file=${file_org%.ll}-inseltpoison.ll
cp $file_org $file
sed -i -E 's/^([^;]*)insertelement <(.*)> undef/\1insertelement <\2> poison/g' $file
head -1 $file | grep "Assertions have been autogenerated by utils/update_test_checks.py" -q
if [ "$?" == 1 ]; then
echo "$file : should be manually updated"
# I manually updated the script
exit 1
fi
python3 ./llvm/utils/update_test_checks.py --opt-binary=./build-releaseassert/bin/opt $file
The transform wasn't checking that the LHS of the comparison
*is* the `X` in question...
This is the miscompile that was holding up D87188.
Thanks to Dave Green for producing an actionable reproducer!
.. because it causes miscompilation when combined with select i1 -> and/or.
It is the select fold which is incorrect; but it is costly to disable the fold, so hack this one.
D92270
Folding a select of vector constants that include undef elements only
applies to fixed vectors, but there's no earlier check the type is not
scalable so it crashes for scalable vectors. This adds a check so this
optimization is only attempted for fixed vectors.
Reviewed By: sdesmalen
Differential Revision: https://reviews.llvm.org/D92046
This should be a perfectly reasonable operation for scalable vectors.
Currently, it only works for zeroinitializer values of
ScalableVectorType, but the fundamental operation is sound and it should
be possible to make it work for other splats
Reviewed By: david-arm
Differential Revision: https://reviews.llvm.org/D77442
This extends D78430 to solve cases like:
https://llvm.org/PR47858
There are still missed opportunities shown in the tests,
and as noted in the earlier patches, we have related
functionality in InstCombine, so we may want to extend
other folds in a similar way.
A semi-random sampling of test diff proofs in this patch:
https://rise4fun.com/Alive/sS4C
As discussed in D89952,
instcombine can sometimes find a way to reduce similar patterns,
but it is incomplete.
InstSimplify uses the computeConstantRange() ValueTracking analysis
via simplifyICmpWithConstant(), so we just need to fill in the max
value of cttz to process any "icmp pred cttz(X), C" pattern (the
min value is initialized to zero automatically).
https://alive2.llvm.org/ce/z/Z_SLWZ
Follow-up to D89976.
As discussed in D89952,
instcombine can sometimes find a way to reduce similar patterns,
but it is incomplete.
InstSimplify uses the computeConstantRange() ValueTracking analysis
via simplifyICmpWithConstant(), so we just need to fill in the max
value of ctlz to process any "icmp pred ctlz(X), C" pattern (the
min value is initialized to zero automatically).
Follow-up to D89976.
As discussed in D89952,
instcombine can sometimes find a way to reduce similar patterns,
but it is incomplete.
InstSimplify uses the computeConstantRange() ValueTracking analysis
via simplifyICmpWithConstant(), so we just need to fill in the max
value of ctpop to process any "icmp pred ctpop(X), C" pattern (the
min value is initialized to zero automatically).
Differential Revision: https://reviews.llvm.org/D89976
This improves simplifications for pattern `icmp (X+Y), (X+Z)` -> `icmp Y,Z`
if only one of the operands has NSW set, e.g.:
icmp slt (x + 0), (x +nsw 1)
We can still safely rewrite this to:
icmp slt 0, 1
because we know that the LHS can't overflow if the RHS has NSW set and
C1 < C2 && C1 >= 0, or C2 < C1 && C1 <= 0
This simplification is useful because ScalarEvolutionExpander which is used to
generate code for SCEVs in different loop optimisers is not always able to put
back NSW flags across control-flow, thus inhibiting CFG simplifications.
Differential Revision: https://reviews.llvm.org/D89317
This patch adds metadata !noundef and makes load instructions can optionally have it.
A load with !noundef always return a well-defined value (has no undef bit or isn't poison).
If the loaded value isn't well defined, the behavior is undefined.
This metadata can be used to encode the assumption from C/C++ that certain reads of variables should have well-defined values.
It is helpful for optimizing freeze instructions away, because freeze can be removed when its operand has well-defined value, and showing that a load from arbitrary location is well-defined is usually hard otherwise.
The same information can be encoded with llvm.assume with operand bundle; using metadata is chosen because I wasn't sure whether code motion can be freely done when llvm.assume is inserted from clang instead.
The existing codebase already is stripping unknown metadata when doing code motion, so using metadata is UB-safe as well.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D89050
This is an alternate fix (see D87835) for a bug where a NaN constant
gets wrongly transformed into Infinity via truncation.
In this patch, we uniformly convert any SNaN to QNaN while raising
'invalid op'.
But we don't have a way to directly specify a 32-bit SNaN value in LLVM IR,
so those are always encoded/decoded by calling convert from/to 64-bit hex.
See D88664 for a clang fix needed to allow this change.
Differential Revision: https://reviews.llvm.org/D88238
As discussed in D87877, instcombine already has this fold,
but it was missing from the more general ValueTracking logic.
https://alive2.llvm.org/ce/z/PumYZP
We shift the significand right on a truncation, but that needs to be made NaN-safe:
always set at least 1 bit in the significand.
https://llvm.org/PR43907
See D88238 for the likely follow-up (but needs some plumbing fixes before it can proceed).
Differential Revision: https://reviews.llvm.org/D87835
-debug-pass is a legacy PM only option.
Some tests checks that the pass returned that it made a change,
which is not relevant to the NPM, since passes return PreservedAnalyses.
Some tests check that passes are freed at the proper time, which is also
not relevant to the NPM.
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D87945
This pass is like DeadCodeEliminationPass, but only does one pass
through a function instead of iterating on users of eliminated
instructions.
DeadCodeEliminationPass should be used in all cases.
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D87933
The output here may not be optimal (yet), but it should be
consistent for commuted operands (it was not before) and
correct. We can do better by checking FMF and NaN if needed.
Code in InstSimplify generally assumes that we have already
folded code like this, so it was not handling 2 constant
inputs by commuting consistently.
If the constant operand is the opposite of the min/max value,
then the result must be the other value.
This is based on the similar codegen transform proposed in:
D87571
As discussed in the sibling codegen functionality patch D87571,
this transform was created with D52766, but it is not correct.
The incorrect test diffs were missed during review, but the
'TODO' comment about this functionality was still in the code -
we need 'nnan' to enable this fold.
This implements support for isKnownNonZero, computeKnownBits when freeze is involved.
```
br (x != 0), BB1, BB2
BB1:
y = freeze x
```
In the above program, we can say that y is non-zero. The reason is as follows:
(1) If x was poison, `br (x != 0)` raised UB
(2) If x was fully undef, the branch again raised UB
(3) If x was non-zero partially undef, say `undef | 1`, `freeze x` will return a nondeterministic value which is also non-zero.
(4) If x was just a concrete value, it is trivial
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D75808
If we know that the abs operand is known negative, we can replace
it with a neg.
To avoid computing known bits twice, I've removed the fold for the
non-negative case from InstSimplify. Both the non-negative and the
negative case are handled by InstCombine now, with one known bits call.
Differential Revision: https://reviews.llvm.org/D87196
This addresses the remaining issue from D87188. Due to a series of
folds, we may end up with abs-of-abs represented as
x == 0 ? -abs(x) : abs(x). Rather than recognizing this as a special
abs pattern and doing an abs-of-abs fold on it afterwards,
I'm directly folding this to one of the select operands in InstSimplify.
The general pattern falls into the "select with operand replaced"
category, but that fold is not powerful enough to recognize that
both hands of the select are the same for value zero.
Differential Revision: https://reviews.llvm.org/D87197
If we have a dominating condition that x >= y, then umax(x, y) is x,
etc. I'm doing this in InstSimplify as the corresponding transform
for the select form is also done there.
Differential Revision: https://reviews.llvm.org/D87168
As discussed in D86843, -earlycse-debug-hash should be used in more regression
tests to catch inconsistency between the hashing and the equivalence check.
Differential Revision: https://reviews.llvm.org/D86863
Replace the check for poison-producing instructions in
SimplifyWithOpReplaced() with the generic helper canCreatePoison()
that properly handles poisonous shifts and thus avoids the problem
from PR47322.
This additionally fixes a bug in IIQ.UseInstrInfo=false mode, which
previously could have caused this code to ignore poison flags.
Setting UseInstrInfo=false should reduce the possible optimizations,
not increase them.
This is not a full solution to the problem, as poison could be
introduced more indirectly. This is just a minimal, easy to backport
fix.
Differential Revision: https://reviews.llvm.org/D86834
Apparently, we don't do this, neither in EarlyCSE, nor in InstSimplify,
nor in (old) GVN, but do in NewGVN and SimplifyCFG of all places..
While i could teach EarlyCSE how to hash PHI nodes,
we can't really do much (anything?) even if we find two identical
PHI nodes in different basic blocks, same-BB case is the interesting one,
and if we teach InstSimplify about it (which is what i wanted originally,
https://reviews.llvm.org/D86530), we get EarlyCSE support for free.
So i would think this is pretty uncontroversial.
On vanilla llvm test-suite + RawSpeed, this has the following effects:
```
| statistic name | baseline | proposed | Δ | % | \|%\| |
|----------------------------------------------------|-----------|-----------|-------:|---------:|---------:|
| instsimplify.NumPHICSE | 0 | 23779 | 23779 | 0.00% | 0.00% |
| asm-printer.EmittedInsts | 7942328 | 7942392 | 64 | 0.00% | 0.00% |
| assembler.ObjectBytes | 273069192 | 273084704 | 15512 | 0.01% | 0.01% |
| correlated-value-propagation.NumPhis | 18412 | 18539 | 127 | 0.69% | 0.69% |
| early-cse.NumCSE | 2183283 | 2183227 | -56 | 0.00% | 0.00% |
| early-cse.NumSimplify | 550105 | 542090 | -8015 | -1.46% | 1.46% |
| instcombine.NumAggregateReconstructionsSimplified | 73 | 4506 | 4433 | 6072.60% | 6072.60% |
| instcombine.NumCombined | 3640264 | 3664769 | 24505 | 0.67% | 0.67% |
| instcombine.NumDeadInst | 1778193 | 1783183 | 4990 | 0.28% | 0.28% |
| instcount.NumCallInst | 1758401 | 1758799 | 398 | 0.02% | 0.02% |
| instcount.NumInvokeInst | 59478 | 59502 | 24 | 0.04% | 0.04% |
| instcount.NumPHIInst | 330557 | 330533 | -24 | -0.01% | 0.01% |
| instcount.TotalInsts | 8831952 | 8832286 | 334 | 0.00% | 0.00% |
| simplifycfg.NumInvokes | 4300 | 4410 | 110 | 2.56% | 2.56% |
| simplifycfg.NumSimpl | 1019808 | 999607 | -20201 | -1.98% | 1.98% |
```
I.e. it fires ~24k times, causes +110 (+2.56%) more `invoke` -> `call`
transforms, and counter-intuitively results in *more* instructions total.
That being said, the PHI count doesn't decrease that much,
and looking at some examples, it seems at least some of them
were previously getting PHI CSE'd in SimplifyCFG of all places..
I'm adjusting `Instruction::isIdenticalToWhenDefined()` at the same time.
As a comment in `InstCombinerImpl::visitPHINode()` already stated,
there are no guarantees on the ordering of the operands of a PHI node,
so if we just naively compare them, we may false-negatively say that
the nodes are not equal when the only difference is operand order,
which is especially important since the fold is in InstSimplify,
so we can't rely on InstCombine sorting them beforehand.
Fixing this for the general case is costly (geomean +0.02%),
and does not appear to catch anything in test-suite, but for
the same-BB case, it's trivial, so let's fix at least that.
As per http://llvm-compile-time-tracker.com/compare.php?from=04879086b44348cad600a0a1ccbe1f7776cc3cf9&to=82bdedb888b945df1e9f130dd3ac4dd3c96e2925&stat=instructions
this appears to cause geomean +0.03% compile time increase (regression),
but geomean -0.01%..-0.04% code size decrease (improvement).
As discussed in
http://lists.llvm.org/pipermail/llvm-dev/2020-July/143801.html.
Currently no users outside of unit tests.
Replace all instances in tests of -constprop with -instsimplify.
Notable changes in tests:
* vscale.ll - @llvm.sadd.sat.nxv16i8 is evaluated by instsimplify, use a fake intrinsic instead
* InsertElement.ll - insertelement undef is removed by instsimplify in @insertelement_undef
llvm/test/Transforms/ConstProp moved to llvm/test/Transforms/InstSimplify/ConstProp
Reviewed By: lattner, nikic
Differential Revision: https://reviews.llvm.org/D85159
This is a reboot of D84655, now performing the inner icmp
simplification query without undef folds.
It should be possible to handle the current foldMinMaxSharedOp()
fold based on this, by moving the logic into icmp of min/max instead,
making it more general. We can't drop the folds for constant operands,
because those also allow undef, which we exclude here.
The tests use assumes for exhaustive coverage, and have a few
more examples of misc folds we get based on icmp simplification.
Differential Revision: https://reviews.llvm.org/D85929
InstSimplify should do all transformations that ConstProp does, but
one thing that ConstProp does that InstSimplify wouldn't is inline
vector instructions that are constants, e.g. into a ret.
Previously vector instructions wouldn't be inlined in InstSimplify
because llvm::Simplify*Instruction() would return nullptr for specific
instructions, such as vector instructions that were actually constants,
if it couldn't simplify them.
This changes SimplifyInsertElementInst, SimplifyExtractElementInst, and
SimplifyShuffleVectorInst to return a vector constant when possible.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D85946
This patch helps getGuaranteedNonPoisonOp find multiple non-poison operands.
Instead of special-casing llvm.assume, I think it is also a viable option to
add noundef to Intrinsics.td. If it makes sense, I'll make a patch for that.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D86477
Previously ConstantFoldExtractElementInstruction() would only work with
insertelement instructions, not contants. This properly handles
insertelement constants as well.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D85865
This recommits the following patches now that D85684 has landed
1cf6f210a2 [IR] Disable select ? C : undef -> C fold in ConstantFoldSelectInstruction unless we know C isn't poison.
469da663f2 [InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably not poison
122b0640fc [InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison
ac0af12ed2 [InstSimplify] Add test cases for opportunities to fold select ?, X, undef -> X when we can prove X isn't poison
9b1e95329a [InstSimplify] Remove select ?, undef, X -> X and select ?, X, undef -> X transforms
I think this is the last remaining translation of an existing
instcombine transform for the corresponding cmp+sel idiom.
This interpretation is more general though - we can remove
mismatched signed/unsigned combinations in addition to the
more obvious cases.
min/max(X, Y) must produce X or Y as the result, so this is
just another clause in the existing transform that was already
matching a min/max of min/max.
I skimmed the existing users of these matchers and don't see any problems
(eg, the caller assumes the matched value was a select instruction without checking).
So I think we can generalize the matching to allow the new intrinsics or the cmp+select idioms.
I did not find any unit tests for the matchers, so added some basics there. The instsimplify
tests are adapted from existing tests for the cmp+select pattern and cover the folds in
simplifyICmpWithMinMax().
Differential Revision: https://reviews.llvm.org/D85230
https://rise4fun.com/Alive/pZEr
Name: mul nuw with icmp eq
Pre: (C2 %u C1) != 0
%a = mul nuw i8 %x, C1
%r = icmp eq i8 %a, C2
=>
%r = false
Name: mul nuw with icmp ne
Pre: (C2 %u C1) != 0
%a = mul nuw i8 %x, C1
%r = icmp ne i8 %a, C2
=>
%r = true
There are potentially several other transforms we need to add based on:
D51625
...but it doesn't look like there was follow-up to that patch.
This revision adds the following peephole optimization
and it's negation:
%a = urem i64 %x, %y
%b = icmp ule i64 %a, %x
====>
%b = true
With John Regehr's help this optimization was checked with Alive2
which suggests it should be valid.
This pattern occurs in the bound checks of Rust code, the program
const N: usize = 3;
const T = u8;
pub fn split_mutiple(slice: &[T]) -> (&[T], &[T]) {
let len = slice.len() / N;
slice.split_at(len * N)
}
the method call slice.split_at will check that len * N is within
the bounds of slice, this bounds check is after some transformations
turned into the urem seen above and then LLVM fails to optimize it
any further. Adding this optimization would cause this bounds check
to be fully optimized away.
ref: https://github.com/rust-lang/rust/issues/74938
Differential Revision: https://reviews.llvm.org/D85092
This is based on the existing code for the non-intrinsic idioms
in InstCombine.
The vector constant constraint is non-obvious: undefs should be
ok in the outer call, but they can't propagate safely from the
inner call in all cases. Example:
https://alive2.llvm.org/ce/z/-2bVbM
define <2 x i8> @src(<2 x i8> %x) {
%0:
%m = umin <2 x i8> %x, { 7, undef }
%m2 = umin <2 x i8> { 9, 9 }, %m
ret <2 x i8> %m2
}
=>
define <2 x i8> @tgt(<2 x i8> %x) {
%0:
%m = umin <2 x i8> %x, { 7, undef }
ret <2 x i8> %m
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
<2 x i8> %x = < undef, undef >
Source:
<2 x i8> %m = < #x00 (0) [based on undef value], #x00 (0) >
<2 x i8> %m2 = < #x00 (0), #x00 (0) >
Target:
<2 x i8> %m = < #x07 (7), #x10 (16) >
Source value: < #x00 (0), #x00 (0) >
Target value: < #x07 (7), #x10 (16) >
It's always safe to pick the earlier abs regardless of the nsw flag. We'll just lose it if it is on the outer abs but not the inner abs.
Differential Revision: https://reviews.llvm.org/D85053
abs() should be rare enough that using value tracking is not going
to be a compile-time cost burden, so use it to reduce a variety of
potential patterns. We do this in DAGCombiner too.
Differential Revision: https://reviews.llvm.org/D85043
This is the main icmp simplification shortcoming seen in D84655.
Alive2 agrees that the basic examples are correct at least:
define <2 x i1> @src(<2 x i8> %x) {
%0:
%r = icmp sle <2 x i8> { undef, 128 }, %x
ret <2 x i1> %r
}
=>
define <2 x i1> @tgt(<2 x i8> %x) {
%0:
ret <2 x i1> { 1, 1 }
}
Transformation seems to be correct!
define <2 x i1> @src(<2 x i32> %X) {
%0:
%A = or <2 x i32> %X, { 63, 63 }
%B = icmp ult <2 x i32> %A, { undef, 50 }
ret <2 x i1> %B
}
=>
define <2 x i1> @tgt(<2 x i32> %X) {
%0:
ret <2 x i1> { 0, 0 }
}
Transformation seems to be correct!
https://alive2.llvm.org/ce/z/omt2eehttps://alive2.llvm.org/ce/z/GW4nP_
Differential Revision: https://reviews.llvm.org/D84762
This is a simple patch that makes canCreateUndefOrPoison use
Instruction::isBinaryOp because BinaryOperator inherits Instruction.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D84596
.. in isGuaranteedNotToBeUndefOrPoison.
This caused early exit of isGuaranteedNotToBeUndefOrPoison, making it return
imprecise result.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D84251
This is a step towards trying to remove unnecessary FP compares
with infinity when compiling with -ffinite-math-only or similar.
I'm intentionally not checking FMF on the fcmp itself because
I'm assuming that will go away eventually.
The analysis part of this was added with rGcd481136 for use with
isKnownNeverNaN. Similarly, that could be an enhancement here to
get predicates like 'one' and 'ueq'.
Differential Revision: https://reviews.llvm.org/D84035
The IR doesn't have a proper concept of invalid pointers, and "null"
constants are just all zeros (though it really needs one).
I think it's not possible to break this for AMDGPU due to the copy
semantics of byval. If you have an original stack object at 0, the
byval copy will be placed above it so I don't think it's really
possible to hit a 0 address.
This reverts most of the following patches due to reports of miscompiles.
I've left the added test cases with comments updated to be FIXMEs.
1cf6f210a2 [IR] Disable select ? C : undef -> C fold in ConstantFoldSelectInstruction unless we know C isn't poison.
469da663f2 [InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably not poison
122b0640fc [InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison
ac0af12ed2 [InstSimplify] Add test cases for opportunities to fold select ?, X, undef -> X when we can prove X isn't poison
9b1e95329a [InstSimplify] Remove select ?, undef, X -> X and select ?, X, undef -> X transforms
Follow up from the transform being removed in D83360. If X is probably not poison, then the transform is safe.
Still plan to remove or adjust the code from ConstantFolding after this.
Differential Revision: https://reviews.llvm.org/D83440
We can't fold to the non-undef value unless we know it isn't poison. So check each element with isGuaranteedNotToBeUndefOrPoison. This currently rules out all constant expressions.
Differential Revision: https://reviews.llvm.org/D83442
Nikita Popov