From 5d63aca604713973ef6079deb0da247efa0e077a Mon Sep 17 00:00:00 2001
From: Dana Robinson <43805+derobins@users.noreply.github.com>
Date: Mon, 8 Aug 2022 12:04:01 -0700
Subject: [PATCH] Turns off clang optimizations on a couple of onion VFD
 functions (#1976)

Ancient clang (7.7.3 on OS X) emits problematic code with -O2 or -O3
---
 src/H5FDonion_history.c |  2 +-
 src/H5FDonion_index.c   |  2 +-
 src/H5private.h         | 13 +++++++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/H5FDonion_history.c b/src/H5FDonion_history.c
index 501a1f7f2f..ed45dddc89 100644
--- a/src/H5FDonion_history.c
+++ b/src/H5FDonion_history.c
@@ -154,7 +154,7 @@ done:
  *              Failure:    0
  *-----------------------------------------------------------------------------
  */
-size_t
+size_t H5_ATTR_NO_OPTIMIZE
 H5FD__onion_history_decode(unsigned char *buf, H5FD_onion_history_t *history)
 {
     uint32_t       ui32        = 0;
diff --git a/src/H5FDonion_index.c b/src/H5FDonion_index.c
index 90eaf0ef16..9dbdde713f 100644
--- a/src/H5FDonion_index.c
+++ b/src/H5FDonion_index.c
@@ -583,7 +583,7 @@ H5FD__onion_revision_index_find(const H5FD_onion_revision_index_t *rix, uint64_t
  *              Failure:    0
  *-----------------------------------------------------------------------------
  */
-size_t
+size_t H5_ATTR_NO_OPTIMIZE
 H5FD__onion_revision_record_decode(unsigned char *buf, H5FD_onion_revision_record_t *record)
 {
     uint32_t       ui32         = 0;
diff --git a/src/H5private.h b/src/H5private.h
index 3130bb1b16..19c3d332a2 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -289,6 +289,18 @@
 #       define H5_ATTR_MALLOC /*void*/
 #  endif
 
+/* Turns off optimizations for a function. Goes after the return type.
+ * Not generally needed in the library, but ancient versions of clang
+ * (7.3.3, possibly others) have trouble with some of the onion VFD decode
+ * functions and need the optimizer turned off. This macro can go away when
+ * we figure out what's going on and can engineer another solution.
+ */
+#  if defined(__clang__)
+#       define H5_ATTR_NO_OPTIMIZE __attribute__((optnone))
+#  else
+#       define H5_ATTR_NO_OPTIMIZE /*void*/
+#  endif
+
 #else
 #   define H5_ATTR_FORMAT(X, Y, Z) /*void*/
 #   define H5_ATTR_UNUSED          /*void*/
@@ -302,6 +314,7 @@
 #   define H5_ATTR_PURE            /*void*/
 #   define H5_ATTR_FALLTHROUGH     /*void*/
 #   define H5_ATTR_MALLOC          /*void*/
+#   define H5_ATTR_NO_OPTIMIZE     /*void*/
 #endif
 /* clang-format on */