add a note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90428 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 32 insertions, 0 deletions

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 2d8a687..2d513c8 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1704,3 +1704,35 @@ need all but the bottom two bits from %A, and if we gave that mask to SDB it
 would delete the or instruction for us.
 
 //===---------------------------------------------------------------------===//
+
+FunctionAttrs is not marking this function as readnone (just readonly):
+$ clang t.c -emit-llvm -S -o - -O0 | opt -mem2reg -S -functionattrs
+
+int t(int a, int b, int c) {
+ int *p;
+ if (a)
+   p = &a;
+ else
+   p = &c;
+ return *p;
+}
+
+This is because we codegen this to:
+
+define i32 @t(i32 %a, i32 %b, i32 %c) nounwind readonly ssp {
+entry:
+  %a.addr = alloca i32                            ; <i32*> [#uses=3]
+  %c.addr = alloca i32                            ; <i32*> [#uses=2]
+...
+
+if.end:
+  %p.0 = phi i32* [ %a.addr, %if.then ], [ %c.addr, %if.else ]
+  %tmp2 = load i32* %p.0                          ; <i32> [#uses=1]
+  ret i32 %tmp2
+}
+
+And functionattrs doesn't realize that the p.0 load points to function local
+memory.
+
+//===---------------------------------------------------------------------===//
+