Otherwise, I’m looking for guidance on what’s needed to land this.
IIRC the paper talks about a call to an unknown function having the desired
semantics. I’ll go with that, making it not throwing, const but looping and
leaf. We can make it stronger if necessary, weakening it after people rely on
semantics is less useful IMO. On RTL we have nothing at all, I’ve heard
,volatile‘ asm is only about missing outputs, not side effects in general. So
I’d expand to nothing on RTL, watching what breaks.
with a follow up to a query
I suggest to remove the assert or replace it with one that checks that if
ECF_LOOPING_CONST_OR_PURE is set the function is either ECF_CONST or ECF_PURE.
Done and re-tested on x86_64-darwin24 and powerpc64le-linux, OK for trunk now?
thanks
Iain
--- 8< ---
P1494 provides a mechanism that serves to demarc epochs within the code
preventing UB-based optimisations from 'time traveling' across such
boundaries. The additional paper, P3641, alters the name of the function
to 'observable_checkpoint' which is the name used here.
This implementation maintains the observable function call through to
expand, where it produces no code.
PR c++/119060
gcc/ChangeLog:
* builtins.cc (expand_builtin): Handle BUILT_IN_OBSERVABLE_CHKPT.
* builtins.def (BUILT_IN_OBSERVABLE_CHKPT): New.
* tree.cc (build_common_builtin_nodes): Build observable
checkpoint builtin.
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_fold): Handle std::observable_checkpoint
specially, lowering it to the builtin.
* cxxapi-data.csv: Add observable_checkpoint to <utility>.
* std-name-hint.gperf: Add observable_checkpoint to <utility>.
* std-name-hint.h: Regenerate.
gcc/testsuite/ChangeLog:
* g++.dg/cpp26/observable-checkpoint.C: New test.
Signed-off-by: Iain Sandoe <[email protected]>
---
gcc/builtins.cc | 4 +
gcc/builtins.def | 1 +
gcc/cp/cp-gimplify.cc | 16 ++
gcc/cp/cxxapi-data.csv | 1 +
gcc/cp/std-name-hint.gperf | 1 +
gcc/cp/std-name-hint.h | 142 +++++++++---------
.../g++.dg/cpp26/observable-checkpoint.C | 25 +++
gcc/tree.cc | 9 +-
8 files changed, 128 insertions(+), 71 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp26/observable-checkpoint.C
diff --git a/gcc/builtins.cc b/gcc/builtins.cc
index 78b561529f5..fb294ce58cd 100644
--- a/gcc/builtins.cc
+++ b/gcc/builtins.cc
@@ -8427,6 +8427,10 @@ expand_builtin (tree exp, rtx target, rtx subtarget,
machine_mode mode,
expand_builtin_unreachable ();
return const0_rtx;
+ case BUILT_IN_OBSERVABLE_CHKPT:
+ /* Generate no code. */
+ return const0_rtx;
+
CASE_FLT_FN (BUILT_IN_SIGNBIT):
case BUILT_IN_SIGNBITD32:
case BUILT_IN_SIGNBITD64:
diff --git a/gcc/builtins.def b/gcc/builtins.def
index 3dc2333c6f2..7cd5353bcb1 100644
--- a/gcc/builtins.def
+++ b/gcc/builtins.def
@@ -1142,6 +1142,7 @@ DEF_C2Y_BUILTIN (BUILT_IN_ULABS, "ulabs",
BT_FN_ULONG_LONG, ATTR_CONST_NO
DEF_C2Y_BUILTIN (BUILT_IN_ULLABS, "ullabs", BT_FN_ULONGLONG_LONGLONG,
ATTR_CONST_NOTHROW_LEAF_LIST)
DEF_GCC_BUILTIN (BUILT_IN_UNREACHABLE_TRAP, "unreachable trap",
BT_FN_VOID, ATTR_CONST_NORETURN_NOTHROW_LEAF_COLD_LIST)
DEF_GCC_BUILTIN (BUILT_IN_UNREACHABLE, "unreachable", BT_FN_VOID,
ATTR_CONST_NORETURN_NOTHROW_LEAF_COLD_LIST)
+DEF_GCC_BUILTIN (BUILT_IN_OBSERVABLE_CHKPT, "observable_checkpoint",
BT_FN_VOID, ATTR_NOTHROW_LEAF_LIST)
DEF_GCC_BUILTIN (BUILT_IN_UNWIND_INIT, "unwind_init", BT_FN_VOID,
ATTR_NULL)
DEF_GCC_BUILTIN (BUILT_IN_UPDATE_SETJMP_BUF, "update_setjmp_buf",
BT_FN_VOID_PTR, ATTR_NULL)
DEF_GCC_BUILTIN (BUILT_IN_VA_COPY, "va_copy",
BT_FN_VOID_VALIST_REF_VALIST_ARG, ATTR_NOTHROW_LEAF_LIST)
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 1662336e165..483d964bd62 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -3452,6 +3452,22 @@ cp_fold (tree x, fold_flags_t flags)
break;
}
+ /* We need to convert std::observable_checkpoint () into its equivalent
+ builtin for correctness, but must not rely on middle-end inlining
+ to do this; special-case it here. */
