This patch disallows load insts from being salvaged. The reason why is PR40628  -- LLVM is unable to determine where the validity of a debug expression containing DW_OP_deref ceases (i.e., where a store overwrites the intended data). It's also (I believe) unable to determine when hoisted/sunk memory instructions would invalidate a dbg.value. This has a large potential for misleading developers.
IMHO this isn't something that can be fixed without a lot of work, thus it's safer to turn off for now. If that's a contentious position though we can find a different way forwards. As mentioned in the PR, the effect on a clang-3.4 build is a 1.1% drop in variable location coverage and 1% drop in scope bytes coverage -- unfortunate, but IMHO not devastating. (I tend to put a lot more weight on avoiding inaccurate DebugInfo, as that diminishes trust in the accurate parts).
Summary of test changes:
- GVN test: -debugify option was added to test that instructions were salvaged in https://reviews.llvm.org/rL325063 . There's another file with -debugify checks added in that commit, so there's still coverage despite me deleting it in this test.
- DeadStoreElimination: tricky as memory operations are exactly what we're messing with. I've altered the test to have an intermediate addition that gets deleted by DSE as being redundant, and added a check that that's salvaged, which appears to have been the original aim of the test.
- InstCombine: this specifically tests for salvaging loads, so must be disabled. I've left a negative test there to fire if this feature is ever re-enabled in a safer condition.
The new test case is just a regression test so that there's a PR available if this behaviour comes back in the future.