[ARM] Follow AACPS for preserving number of loads/stores of volatile bit-fields
Summary:
Following the AAPCS, every store to a volatile bit-field requires to generate one load of that field, even if all the bits are going to be replaced.
This patch allows the user to opt-in in following such rule, whenever the a.
AAPCS Release 2019Q1.1 (https://static.docs.arm.com/ihi0042/g/aapcs32.pdf)
section 8.1 Data Types, page 35, paragraph: Volatile bit-fields – preserving number and width of container accesses
```
When a volatile bit-field is written, and its container does not overlap with any non-bit-field member, its
container must be read exactly once and written exactly once using the access width appropriate to the
type of the container. The two accesses are not atomic.
```
Reviewers: lebedev.ri, ostannard, jfb, eli.friedman
Reviewed By: jfb
Subscribers: rsmith, rjmccall, dexonsmith, kristof.beyls, jfb, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D67399
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 17fc16a..4101ca2 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -415,6 +415,11 @@
llvm_unreachable("unknown storage duration");
}
+/// Helper method to check if the underlying ABI is AAPCS
+static bool isAAPCS(const TargetInfo &TargetInfo) {
+ return TargetInfo.getABI().startswith("aapcs");
+}
+
LValue CodeGenFunction::
EmitMaterializeTemporaryExpr(const MaterializeTemporaryExpr *M) {
const Expr *E = M->getSubExpr();
@@ -2068,6 +2073,14 @@
SrcVal = Builder.CreateOr(Val, SrcVal, "bf.set");
} else {
assert(Info.Offset == 0);
+ // According to the AACPS:
+ // When a volatile bit-field is written, and its container does not overlap
+ // with any non-bit-field member, its container must be read exactly once and
+ // written exactly once using the access width appropriate to the type of the
+ // container. The two accesses are not atomic.
+ if (Dst.isVolatileQualified() && isAAPCS(CGM.getTarget()) &&
+ CGM.getCodeGenOpts().ForceAAPCSBitfieldLoad)
+ Builder.CreateLoad(Ptr, true, "bf.load");
}
// Write the new value back out.