[PATCH] RDMA/irdma: Slightly optimize irdma_form_ah_cm_frame()

[Christophe JAILLET]

There is no need to zero 'pktsize' bytes of 'buf', only the header needs
to be cleared, to be safe.
All the other bytes are already written with some memcpy() at the end of
the function.

Doing so also gives the opportunity to the compiler to avoid the memset()
call. It can be inlined now that the length is known as compile time.

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
Just in case, here is the diff of what is generated by gcc 11.3.0 before
and after the patch.

 .L736:
-# drivers/infiniband/hw/irdma/cm.c:340: 	memset(buf, 0, pktsize);
+# drivers/infiniband/hw/irdma/cm.c:340: 	memset(buf, 0, sizeof(*tcph));
 	call	__sanitizer_cov_trace_pc	#
-	xorl	%esi, %esi	#
-	movzwl	%r13w, %edx	# _194, __fortify_size
-	movq	%rbp, %rdi	# buf,
-	call	memset	#
-	leaq	104(%r12), %rax	#, _259
+	movl	$0, 16(%rbp)	#, MEM <char[1:20]> [(void *)buf_114]
+	leaq	104(%r12), %rax	#, _295
+# drivers/infiniband/hw/irdma/cm.c:342: 	sqbuf->totallen = pktsize;
+	movzwl	%r13w, %r13d	# _192, _192
+# drivers/infiniband/hw/irdma/cm.c:340: 	memset(buf, 0, sizeof(*tcph));
+	movq	$0, 0(%rbp)	#, MEM <char[1:20]> [(void *)buf_114]
+# drivers/infiniband/hw/irdma/cm.c:342: 	sqbuf->totallen = pktsize;
+	movq	%rax, %rdi	# _295,
+# drivers/infiniband/hw/irdma/cm.c:340: 	memset(buf, 0, sizeof(*tcph));
+	movq	$0, 8(%rbp)	#, MEM <char[1:20]> [(void *)buf_114]
+	movq	%rax, 64(%rsp)	# _295, %sfp
 # drivers/infiniband/hw/irdma/cm.c:342: 	sqbuf->totallen = pktsize;
---
 drivers/infiniband/hw/irdma/cm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/irdma/cm.c b/drivers/infiniband/hw/irdma/cm.c
index 195aa9ea18b6..48c2a303e9ec 100644
--- a/drivers/infiniband/hw/irdma/cm.c
+++ b/drivers/infiniband/hw/irdma/cm.c
@@ -337,7 +337,7 @@ static struct irdma_puda_buf *irdma_form_ah_cm_frame(struct irdma_cm_node *cm_no
 
 	pktsize = sizeof(*tcph) + opts_len + hdr_len + pd_len;
 
-	memset(buf, 0, pktsize);
+	memset(buf, 0, sizeof(*tcph));
 
 	sqbuf->totallen = pktsize;
 	sqbuf->tcphlen = sizeof(*tcph) + opts_len;
-- 
2.34.1