Re: [PATCH] create_delta_index: simplify condition always evaluating to true
From: Philip Oakley <hidden>
Date: 2016-06-15 22:58:26
From: "Stefan Beller" <redacted>
When checking the previous lines in that function, we can deduce that hsize must always be smaller than (1u<<31), since 506049c7df2c6 (fix >4GiB source delta assertion failure), because entries is capped at an upper bound of 0xfffffffeU, so hsize contains a maximum value of 0x3fffffff, which is smaller than (1u<<31), so the value of 'i' will never be larger than 31. Signed-off-by: Stefan Beller <redacted> --- Eric, thanks for reviewing my patch. I applied the first 2 proposals (deduce, entries), but I disagree on the third, so I reformulated the sentence, as I really meant the variable i and not it as a pronoun. Do I understand right, you're suggesting to remove the source code comment? I did this now, but I have a bad feeling with it. The change of this patch surely removes dead code as of now and makes it more readable. But also it could become alive again, once somebody changes things nearby and forgets about the assumption, hsize not exceeding a certain size. That's why I put a comment in there, so the future changes nearby may be more careful.
Should the comment also include a note about potential undefined behaviour of a large shift, with the possible consequential bad compiler optimization, such as simply deleting the code. I understand the initial spot was part of the STACK tool check of such latent optimisation bugs. http://css.csail.mit.edu/stack/
quoted hunk ↗ jump to hunk
Thanks, Stefan diff-delta.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/diff-delta.c b/diff-delta.c index 93385e1..3797ce6 100644 --- a/diff-delta.c +++ b/diff-delta.c@@ -155,7 +155,7 @@ struct delta_index * create_delta_index(const void*buf, unsigned long bufsize) entries = 0xfffffffeU / RABIN_WINDOW; } hsize = entries / 4; - for (i = 4; (1u << i) < hsize && i < 31; i++); + for (i = 4; (1u << i) < hsize; i++); hsize = 1 << i; hmask = hsize - 1; -- 1.8.4.rc3.1.gc1ebd90