Fix for PR18102.

Issue outcomes from DAGCombiner::MergeConsequtiveStores, more precisely from
mem-ops sequence sorting.

Consider, how MergeConsequtiveStores works for next example:

store i8 1, a[0]
store i8 2, a[1]
store i8 3, a[1]   ; a[1] again.
return   ; DAG starts here

1. Method will collect all the 3 stores.
2. It sorts them by distance from the base pointer (farthest with highest
index).
3. It takes first consecutive non-overlapping stores and (if possible) replaces
them with a single store instruction.

The point is, we can't determine here which 'store' instruction
would be the second after sorting ('store 2' or 'store 3').
It happens that 'store 3' would be the second, and 'store 2' would be the third.

So after merging we have the next result:

store i16 (1 | 3 << 8), base   ; is a[0] but bit-casted to i16
store i8 2, a[1]

So actually we swapped 'store 3' and 'store 2' and got wrong contents in a[1].

Fix: In sort routine just also take into account mem-op sequence number. 
llvm-svn: 200201

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/Generic/stores-merging.ll b/llvm/test/CodeGen/Generic/stores-merging.ll
new file mode 100644
index 00000000000..61dea088995
--- /dev/null
+++ b/llvm/test/CodeGen/Generic/stores-merging.ll
@@ -0,0 +1,23 @@
+; RUN: llc < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%structTy = type { i8, i32, i32 }
+
+@e = common global %structTy zeroinitializer, align 4
+
+; CHECK-LABEL: f
+define void @f() {
+entry:
+
+; CHECK:   movabsq	$528280977409, %rax
+; CHECK:   movq    %rax, e+4(%rip)
+; CHECK:   movl    $456, e+8(%rip)
+
+  store i32 1, i32* getelementptr inbounds (%structTy* @e, i64 0, i32 1), align 4
+  store i32 123, i32* getelementptr inbounds (%structTy* @e, i64 0, i32 2), align 4
+  store i32 456, i32* getelementptr inbounds (%structTy* @e, i64 0, i32 2), align 4
+  ret void
+}
+