Fix mem2reg to correctly handle allocas only used in a single block

Currently, a load from an alloca that is used in as single block and is not preceded
by a store is replaced by undef. This is not always correct if the single block is
inside a loop.
Fix the logic so that:
1) If there are no stores in the block, replace the load with an undef, as before.
2) If there is a store (regardless of where it is in the block w.r.t the load), bail
out, and let the rest of mem2reg handle this alloca.

Patch by: gil.rapaport@intel.com
Differential Revision: http://reviews.llvm.org/D11355

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242884 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/Mem2Reg/pr24179.ll

@@ -0,0 +1,44 @@
+; RUN: opt -mem2reg < %s -S | FileCheck %s
+
+declare i32 @def(i32)
+declare i1 @use(i32)
+
+; Special case of a single-BB alloca does not apply here since the load
+; is affected by the following store. Expect this case to be identified
+; and a PHI node to be created.
+define void @test1() {
+; CHECK-LABEL: @test1(
+ entry:
+  %t = alloca i32
+  br label %loop
+
+ loop:
+  %v = load i32, i32* %t
+  %c = call i1 @use(i32 %v)
+; CHECK: [[PHI:%.*]] = phi i32 [ undef, %entry ], [ %n, %loop ]
+; CHECK: call i1 @use(i32 [[PHI]])
+  %n = call i32 @def(i32 7)
+  store i32 %n, i32* %t
+  br i1 %c, label %loop, label %exit
+
+ exit:
+  ret void
+}
+
+; Same as above, except there is no following store. The alloca should just be
+; replaced with an undef
+define void @test2() {
+; CHECK-LABEL: @test2(
+ entry:
+  %t = alloca i32
+  br label %loop
+
+ loop:
+  %v = load i32, i32* %t
+  %c = call i1 @use(i32 %v)
+; CHECK: %c = call i1 @use(i32 undef)
+  br i1 %c, label %loop, label %exit
+
+ exit:
+  ret void
+}