btf_loader: Use cacheline size to infer alignment

When the alignment is larger than natural, it is very likely that the
source code was using the cacheline size. Therefore, use the cacheline
size when it would only result in increasing the alignment.

Committer tests:

This is one of the cases that this heuristic works well, 'struct Qdisc'
in the Linux kernel:

  --- /tmp/btfdiff.dwarf.pXdgRU   2021-10-28 10:22:11.738200232 -0300
  +++ /tmp/btfdiff.btf.bkDkdf     2021-10-28 10:22:11.925205061 -0300
  @@ -107,7 +107,7 @@ struct Qdisc {
          /* XXX 24 bytes hole, try to pack */

          /* --- cacheline 2 boundary (128 bytes) --- */
  -       struct sk_buff_head        gso_skb __attribute__((__aligned__(64))); /*   128    24 */
  +       struct sk_buff_head        gso_skb __attribute__((__aligned__(32))); /*   128    24 */
          struct qdisc_skb_head      q;                    /*   152    24 */
          struct gnet_stats_basic_packed bstats;           /*   176    16 */
          /* --- cacheline 3 boundary (192 bytes) --- */

With this patch both DWARF and BTF generated output have the same
alignment.

Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: dwarves@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/btf_loader.c b/btf_loader.c
index e500eaee..7a5b16ff 100644
--- a/btf_loader.c
+++ b/btf_loader.c
@@ -476,6 +476,7 @@ static uint32_t class__infer_alignment(const struct conf_load *conf,
 				       uint32_t natural_alignment,
 				       uint32_t smallest_offset)
 {
+	uint16_t cacheline_size = conf->conf_fprintf->cacheline_size;
 	uint32_t alignment = 0;
 	uint32_t offset_delta = byte_offset - smallest_offset;
 
@@ -494,6 +495,15 @@ static uint32_t class__infer_alignment(const struct conf_load *conf,
 	/* Natural alignment, nothing to do */
 	if (alignment <= natural_alignment || alignment == 1)
 		alignment = 0;
+	/* If the offset is compatible with being aligned on the cacheline size
+	 * and this would only result in increasing the alignment, use the
+	 * cacheline size as it is safe and quite likely to be what was in the
+	 * source.
+	 */
+	else if (alignment < cacheline_size &&
+		 cacheline_size % alignment == 0 &&
+		 byte_offset % cacheline_size == 0)
+		alignment = cacheline_size;
 
 	return alignment;
 }