[PATCH v2 1/3] lib: zstd: Fix unused variable warning

[Nick Terrell <nickrterrell@gmail.com>]

From: Nick Terrell <terrelln@fb.com>

The variable `litLengthSum` is only used by an `assert()`, so when
asserts are disabled the compiler doesn't see any usage and warns.

This issue is already fixed upstream by PR #2838 [0]. It was reported
by the Kernel test robot in [1].

Another approach would be to change zstd's disabled `assert()`
definition to use the argument in a disabled branch, instead of
ignoring the argument. I've avoided this approach because there are
some small changes necessary to get zstd to build, and I would
want to thoroughly re-test for performance, since that is slightly
changing the code in every function in zstd. It seems like a
trivial change, but some functions are pretty sensitive to small
changes. However, I think it is a valid approach that I would
like to see upstream take, so I've opened Issue #2868 to attempt
this upstream.

[0] https://github.com/facebook/zstd/pull/2838
[1] https://lore.kernel.org/linux-mm/202111120312.833wII4i-lkp@intel.com/T/
[2] https://github.com/facebook/zstd/issues/2868

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nick Terrell <terrelln@fb.com>
---
 lib/zstd/compress/zstd_compress_superblock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/zstd/compress/zstd_compress_superblock.c b/lib/zstd/compress/zstd_compress_superblock.c
index ee03e0aedb03..b0610b255653 100644
--- a/lib/zstd/compress/zstd_compress_superblock.c
+++ b/lib/zstd/compress/zstd_compress_superblock.c
@@ -411,6 +411,8 @@ static size_t ZSTD_seqDecompressedSize(seqStore_t const* seqStore, const seqDef*
     const seqDef* sp = sstart;
     size_t matchLengthSum = 0;
     size_t litLengthSum = 0;
+    /* Only used by assert(), suppress unused variable warnings in production. */
+    (void)litLengthSum;
     while (send-sp > 0) {
         ZSTD_sequenceLength const seqLen = ZSTD_getSequenceLength(seqStore, sp);
         litLengthSum += seqLen.litLength;
-- 
2.33.1