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1bc7315c02
This is how it ignores the dead code:
1) When a dead branch target, say block B, is identified, all the
blocks dominated by B is dead as well.
2) The PHIs of those blocks in dominance-frontier(B) is updated such
that the operands corresponding to dead predecessors are replaced
by "UndefVal".
Using lattice's jargon, the "UndefVal" is the "Top" in essence.
Phi node like this "phi(v1 bb1, undef xx)" will be optimized into
"v1" if v1 is constant, or v1 is an instruction which dominate this
PHI node.
3) When analyzing the availability of a load L, all dead mem-ops which
L depends on disguise as a load which evaluate exactly same value as L.
4) The dead mem-ops will be materialized as "UndefVal" during code motion.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191017 91177308-0d34-0410-b5e6-96231b3b80d8
21 lines
358 B
LLVM
21 lines
358 B
LLVM
; RUN: opt < %s -basicaa -gvn -S | grep "DEAD = phi i32 "
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define i32 @main(i32* %p, i32 %x, i32 %y) {
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block1:
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%z = load i32* %p
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%cmp = icmp eq i32 %x, %y
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br i1 %cmp, label %block2, label %block3
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block2:
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br label %block4
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block3:
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%b = bitcast i32 0 to i32
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store i32 %b, i32* %p
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br label %block4
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block4:
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%DEAD = load i32* %p
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ret i32 %DEAD
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}
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