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

1 files changed, 3 insertions, 3 deletions

diff --git a/test/Transforms/GVN/local-pre.ll b/test/Transforms/GVN/local-pre.ll
index 1d0dadfbe0..2c92699dca 100644
--- a/test/Transforms/GVN/local-pre.ll
+++ b/test/Transforms/GVN/local-pre.ll
@@ -1,9 +1,9 @@
 ; RUN: opt < %s -gvn -enable-pre -S | grep "b.pre"
 
-define i32 @main(i32 %p) {
+define i32 @main(i32 %p, i32 %q) {
 block1:
-  
-	br i1 true, label %block2, label %block3
+    %cmp = icmp eq i32 %p, %q 
+	br i1 %cmp, label %block2, label %block3
 
 block2:
  %a = add i32 %p, 1