Re: 1024bb26337b - main - qdivrem: Predict division by zero as false.

-----Original Message-----
From: <owner-src-committers@freebsd.org> on behalf of Hans Petter Selasky <hselasky@FreeBSD.org>
Date: 2022-10-04, Tuesday at 04:53
To: <src-committers@FreeBSD.org>, <dev-commits-src-all@FreeBSD.org>, <dev-commits-src-main@FreeBSD.org>
Subject: git: 1024bb26337b - main - qdivrem: Predict division by zero as false.

    The branch main has been updated by hselasky:

    URL: https://cgit.FreeBSD.org/src/commit/?id=1024bb26337bdc6679af477977247e9155d502bc

    commit 1024bb26337bdc6679af477977247e9155d502bc
    Author:     Hans Petter Selasky <hselasky@FreeBSD.org>
    AuthorDate: 2022-10-04 10:28:25 +0000
    Commit:     Hans Petter Selasky <hselasky@FreeBSD.org>
    CommitDate: 2022-10-04 11:51:06 +0000

        qdivrem: Predict division by zero as false.

        Division by zero triggers an arithmetic exception and should not be very
        common. Predict this.

Looking at this change with a little more context, I see that (static volatile const unsigned int) 'zero' is set to 0... and then used as the denominator for division. Since these are all integers, I would expect that to cause a fatal divide-by-zero exception. And yet, there's more code after that divide-by-zero.

What am I missing?

Thanks,

Ravi (rpokala@)

        No functional change intended.

        MFC after:      1 week
        Sponsored by:   NVIDIA Networking
    ---
     lib/libc/quad/qdivrem.c | 2 +-
     sys/libkern/qdivrem.c   | 2 +-
     2 files changed, 2 insertions(+), 2 deletions(-)

    diff --git a/lib/libc/quad/qdivrem.c b/lib/libc/quad/qdivrem.c
    index ea09e7f43f16..7dd7632d39ed 100644
    --- a/lib/libc/quad/qdivrem.c
    +++ b/lib/libc/quad/qdivrem.c
    @@ -94,7 +94,7 @@ __qdivrem(u_quad_t uq, u_quad_t vq, u_quad_t *arq)
     	/*
     	 * Take care of special cases: divide by zero, and u < v.
     	 */
    -	if (vq == 0) {
    +	if (__predict_false(vq == 0)) {
     		/* divide by zero. */
     		static volatile const unsigned int zero = 0;

    diff --git a/sys/libkern/qdivrem.c b/sys/libkern/qdivrem.c
    index 2429fe708d7b..3f2834166450 100644
    --- a/sys/libkern/qdivrem.c
    +++ b/sys/libkern/qdivrem.c
    @@ -91,7 +91,7 @@ __qdivrem(u_quad_t uq, u_quad_t vq, u_quad_t *arq)
     	/*
     	 * Take care of special cases: divide by zero, and u < v.
     	 */
    -	if (vq == 0) {
    +	if (__predict_false(vq == 0)) {
     		/* divide by zero. */
     		static volatile const unsigned int zero = 0;