Fix PR13969, a mini-phase-ordering issue with the new SROA pass.

Currently, we re-visit allocas when something changes about the way they
might be *split* to allow better scalarization to take place. However,
we weren't handling the case when the *promotion* is what would change
the behavior of SROA. When an address derived from an alloca is stored
into another alloca, we consider the first to have escaped. If the
second is ever promoted to an SSA value, we will suddenly be able to run
the SROA pass on the first alloca.

This patch adds explicit support for this form if iteration. When we
detect a store of a pointer derived from an alloca, we flag the
underlying alloca for reprocessing after promotion. The logic works hard
to only do this when there is definitely going to be promotion and it
might remove impediments to the analysis of the alloca.

Thanks to Nick for the great test case and Benjamin for some sanity
check review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165223 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 24 insertions, 0 deletions

diff --git a/test/Transforms/SROA/basictest.ll b/test/Transforms/SROA/basictest.ll
index b6d08ba8c8..a58eb4786c 100644
--- a/test/Transforms/SROA/basictest.ll
+++ b/test/Transforms/SROA/basictest.ll
@@ -926,3 +926,27 @@ bb3:
 bb4:
   unreachable
 }
+
+define double @PR13969(double %x) {
+; Check that we detect when promotion will un-escape an alloca and iterate to
+; re-try running SROA over that alloca. Without that, the two allocas that are
+; stored into a dead alloca don't get rewritten and promoted.
+; CHECK: @PR13969
+
+entry:
+  %a = alloca double
+  %b = alloca double*
+  %c = alloca double
+; CHECK-NOT: alloca
+
+  store double %x, double* %a
+  store double* %c, double** %b
+  store double* %a, double** %b
+  store double %x, double* %c
+  %ret = load double* %a
+; CHECK-NOT: store
+; CHECK-NOT: load
+
+  ret double %ret
+; CHECK: ret double %x
+}