Variable used uninitialized in libavcodec/ffv1enc.c

[Jeremy Huddleston]

The following were observed when building N-77414-g484cc66 and should probably be fixed:

libavcodec/ffv1enc.c:434:17: warning: variable 'a' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
            if (lbd) {
                ^~~
libavcodec/ffv1enc.c:457:31: note: uninitialized use occurs here
            sample[3][0][x] = a;
                              ^
libavcodec/ffv1enc.c:434:13: note: remove the 'if' if its condition is always true
            if (lbd) {
            ^~~~~~~~~
libavcodec/ffv1enc.c:433:13: note: variable 'a' is declared here
            int b, g, r, av_uninit(a);
            ^

[Carl Eugen Hoyos]

The warning was originally addressed in 0b23452c01c5f8145de111f09c0e9a7d5bd82068
When encoding RGB >8 bit, line 453 reads from an uninitialized value:

sample[3][0][x] = a;

sample[3][0][x] will not be used afterwards.
I suspect this is speed-critical code, a possible workaround would be to manually unroll the for (p = 0; p < 3 + s->transparency; p++) { loop in line 455 and move the above assignment in the transparency part of the unrolled loop. This may still incorrectly trigger the warning though.