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| author | Shuxin Yang <shuxin.llvm@gmail.com> | 2013-09-19 17:22:51 +0000 |
|---|---|---|
| committer | Shuxin Yang <shuxin.llvm@gmail.com> | 2013-09-19 17:22:51 +0000 |
| commit | 1bc7315c022b327a496366a78eb31ba446c699bd (patch) | |
| tree | a253f541fe901a797a2623c361526b8521bce745 /test/Transforms/GVN/2008-07-02-Unreachable.ll | |
| parent | 31eb340cb68172874f5ad6d1fd7b3cb286a8615c (diff) | |
| download | external_llvm-1bc7315c022b327a496366a78eb31ba446c699bd.zip external_llvm-1bc7315c022b327a496366a78eb31ba446c699bd.tar.gz external_llvm-1bc7315c022b327a496366a78eb31ba446c699bd.tar.bz2 | |
GVN proceeds in the presence of dead code.
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
Diffstat (limited to 'test/Transforms/GVN/2008-07-02-Unreachable.ll')
| -rw-r--r-- | test/Transforms/GVN/2008-07-02-Unreachable.ll | 8 |
1 files changed, 3 insertions, 5 deletions
diff --git a/test/Transforms/GVN/2008-07-02-Unreachable.ll b/test/Transforms/GVN/2008-07-02-Unreachable.ll index 4f07868..ce83fa4 100644 --- a/test/Transforms/GVN/2008-07-02-Unreachable.ll +++ b/test/Transforms/GVN/2008-07-02-Unreachable.ll @@ -3,10 +3,11 @@ @g_3 = external global i8 ; <i8*> [#uses=2] -define i8 @func_1() nounwind { +define i8 @func_1(i32 %x, i32 %y) nounwind { entry: %A = alloca i8 - br i1 false, label %ifelse, label %ifthen + %cmp = icmp eq i32 %x, %y + br i1 %cmp, label %ifelse, label %ifthen ifthen: ; preds = %entry br label %ifend @@ -14,9 +15,6 @@ ifthen: ; preds = %entry ifelse: ; preds = %entry %tmp3 = load i8* @g_3 ; <i8> [#uses=0] store i8 %tmp3, i8* %A - br label %forcond.thread - -forcond.thread: ; preds = %ifelse br label %afterfor forcond: ; preds = %forinc |