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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 |
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Analysis | ||
AsmParser | ||
Bitcode | ||
CodeGen | ||
DebugInfo | ||
ExecutionEngine | ||
IR | ||
IRReader | ||
Linker | ||
MC | ||
Object | ||
Option | ||
Support | ||
TableGen | ||
Target | ||
Transforms | ||
CMakeLists.txt | ||
LLVMBuild.txt | ||
Makefile |