[FunctionAttrs] Fix an iterator wraparound bug

Summary:
This change fixes an iterator wraparound bug in
`determinePointerReadAttrs`.

Ideally, ++'ing off the `end()` of an iplist should result in a failed
assert, but currently iplist seems to silently wrap to the head of the
list on `end()++`.  This is why the bad behavior is difficult to
demonstrate.

Reviewers: chandlerc, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14350

llvm-svn: 252386

1 files changed, 30 insertions, 0 deletions

diff --git a/llvm/test/Transforms/FunctionAttrs/out-of-bounds-iterator-bug.ll b/llvm/test/Transforms/FunctionAttrs/out-of-bounds-iterator-bug.ll
new file mode 100644
index 00000000000..db9a895f97e
--- /dev/null
+++ b/llvm/test/Transforms/FunctionAttrs/out-of-bounds-iterator-bug.ll
@@ -0,0 +1,30 @@
+; RUN: opt -functionattrs -S < %s | FileCheck %s
+
+; This checks for an iterator wraparound bug in FunctionAttrs.  The previous
+; "incorrect" behavior was inferring readonly for the %x argument in @caller.
+; Inferring readonly for %x *is* actually correct, since @va_func is marked
+; readonly, but FunctionAttrs was inferring readonly for the wrong reasons (and
+; we _need_ the readonly on @va_func to trigger the problematic code path).  It
+; is possible that in the future FunctionAttrs becomes smart enough to infer
+; readonly for %x for the right reasons, and at that point this test will have
+; to be marked invalid.
+
+declare void @llvm.va_start(i8*)
+declare void @llvm.va_end(i8*)
+
+define void @va_func(i32* readonly %b, ...) readonly nounwind {
+; CHECK-LABEL: define void @va_func(i32* nocapture readonly %b, ...)
+ entry:
+  %valist = alloca i8
+  call void @llvm.va_start(i8* %valist)
+  call void @llvm.va_end(i8* %valist)
+  %x = call i32 @caller(i32* %b)
+  ret void
+}
+
+define i32 @caller(i32* %x) {
+; CHECK-LABEL: define i32 @caller(i32* nocapture %x)
+ entry:
+  call void(i32*,...) @va_func(i32* null, i32 0, i32 0, i32 0, i32* %x)
+  ret i32 42
+}