Because of this, we cannot use the Simplify* APIs, as they can assert-fail on unreachable code. Since it's not easy to determine
if a given threading will cause a block to become unreachable, simply defer simplifying simplification to later InstCombine and/or
DCE passes.
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I have not been able to find a way to test each in isolation, for a few reasons:
1) The ability to look-through non-i1 BinaryOperator's requires the ability to look through non-constant
ICmps in order for it to ever trigger.
2) The ability to do LVI-powered PHI value determination only matters in cases that ProcessBranchOnPHI
can't handle. Since it already handles all the cases without other instructions in the def-use chain
between the PHI and the branch, it requires the ability to look through ICmps and/or BinaryOperators
as well.
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This actually exposed an infinite recursion bug in ComputeValueKnownInPredecessors which theoretically already existed (in JumpThreading's
handling of and/or of i1's), but never manifested before. This patch adds a tracking set to prevent this case.
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any load in the default address space that completes implies that the base value that it GEP'd from
was not null.
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from the LHS should disable reconsidering that pred on the
RHS. However, knowing something about the pred on the RHS
shouldn't disable subsequent additions on the RHS from
happening.
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mutated by recursive simplification. This also enhances
ReplaceAndSimplifyAllUses to actually do a real RAUW
at the end of it, which updates any value handles
pointing to "From" to start pointing to "To". This
seems useful for debug info and random other VH users.
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the LHS and RHS of an and/or instruction, don't multiply add
known predecessor values. This fixes the crash on testcase
from PR7498
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for arbitrary terminators in predecessors, don't assume
it is a conditional or uncond branch. The testcase shows
an example where they can happen with switches.
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handle the case when we can infer an input to the xor
from all inputs that agree, instead of going into an
infinite loop. Another part of PR6199
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in JT.
2) When cloning blocks for PHI or xor conditions, use
instsimplify to simplify the code as we go. This allows us to
squish common cases early in JT which opens up opportunities for
subsequent iterations, and allows it to completely simplify the
testcase.
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condition is a xor with a phi node. This eliminates nonsense
like this from 176.gcc in several places:
LBB166_84:
testl %eax, %eax
- setne %al
- xorb %cl, %al
- notb %al
- testb $1, %al
- je LBB166_85
+ je LBB166_69
+ jmp LBB166_85
This is rdar://7391699
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debug intrinsics, and an unconditional branch when possible. This
reuses the TryToSimplifyUncondBranchFromEmptyBlock function split
out of simplifycfg.
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just one level deep. On the testcase we go from getting this:
F1: ; preds = %T2
%F = and i1 true, %cond ; <i1> [#uses=1]
br i1 %F, label %X, label %Y
to a fully threaded:
F1: ; preds = %T2
br label %Y
This changes gets us to the point where we're forming (too many) switch
instructions on doug's strswitch testcase.
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(making pred factoring only happen if threading is guaranteed
to be successful).
This now survives an X86-64 bootstrap of llvm-gcc.
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predicates. This allows us to jump thread things like:
_ZN12StringSwitchI5ColorE4CaseILj7EEERS1_RAT__KcRKS0_.exit119:
%tmp1.i24166 = phi i8 [ 1, %bb5.i117 ], [ %tmp1.i24165, %_Z....exit ], [ %tmp1.i24165, %bb4.i114 ]
%toBoolnot.i87 = icmp eq i8 %tmp1.i24166, 0 ; <i1> [#uses=1]
%tmp4.i90 = icmp eq i32 %tmp2.i, 6 ; <i1> [#uses=1]
%or.cond173 = and i1 %toBoolnot.i87, %tmp4.i90 ; <i1> [#uses=1]
br i1 %or.cond173, label %bb4.i96, label %_ZN12...
Where it is "obvious" that when coming from %bb5.i117 that the 'and' is always
false. This triggers a surprisingly high number of times in the testsuite,
and gets us closer to generating good code for doug's strswitch testcase.
This also make a bunch of other code in jump threading redundant, I'll rip
out in the next patch. This survived an enable-checking llvm-gcc bootstrap.
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When an incoming value for a PHI is updated, we must also updated all other
incoming values for the same BB to match, otherwise we create invalid PHIs.
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done by condprop, but do it in a much more general form. The
basic idea is that we can do a limited form of tail duplication
in the case when we have a branch on a phi. Moving the branch
up in to the predecessor block makes instruction selection
much easier and encourages chained jump threadings.
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