ghc-prim: hs_atomicread* and hs_atomicwrite* missing barriers

The comments in compiler/prelude/primops.txt.pp for both operations say "... Implies a full memory barrier."

The implementation does not issue any barriers as exemplified by the 32-bit variants as can be seen in the excerpts from libraries/ghc-prim/cbits/atomic.c.

StgWord
hs_atomicread32(StgWord x)
{
  return *(volatile StgWord32 *) x;
}

and

void
hs_atomicwrite32(StgWord x, StgWord val)
{
  *(volatile StgWord32 *) x = (StgWord32) val;
}

The native code generator for X86/amd64 and the LLVM backend do not generate calls to these functions but generate the necessary barrier (mfence) directly and thus are not affected by this issue.

There are no gcc __sync_* intrinsics for the two operations. The new __atomic_* intrinisics have the required operations but require gcc 4.7 or later.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Prelude
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture