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merge-recursive: silence -Wxor-used-as-pow warning

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The merge-recursive code uses stage number constants like this:

  add = &ci->ren1->dst_entry->stages[2 ^ 1];
  ...
  add = &ci->ren2->dst_entry->stages[3 ^ 1];

The xor has the effect of flipping the "1" bit, so that "2 ^ 1" becomes
"3" and "3 ^ 1" becomes "2", which correspond to the "ours" and "theirs"
stages respectively.

Unfortunately, clang-10 and up issue a warning for this code:

  merge-recursive.c:1759:40: error: result of '2 ^ 1' is 3; did you mean '1 << 1' (2)? [-Werror,-Wxor-used-as-pow]
                  add = &ci->ren1->dst_entry->stages[2 ^ 1];
                                                     ~~^~~
                                                     1 << 1
  merge-recursive.c:1759:40: note: replace expression with '0x2 ^ 1' to silence this warning

We could silence it by using 0x2, as the compiler mentions. Or by just
using the constants "2" and "3" directly. But after digging into it, I
do think this bit-flip is telling us something. If we just wrote:

  add = &ci->ren2->dst_entry->stages[2];

for the second one, you might think that "ren2" and "2" correspond. But
they don't. The logic is: ren2 is theirs, which is stage 3, but we
are interested in the opposite side's stage, so flip it to 2.

So let's keep the bit-flipping, but let's also put it behind a named
function, which will make its purpose a bit clearer. This also has the
side effect of suppressing the warning (and an optimizing compiler
should be able to easily turn it into a constant as before).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2020-01-25 00:37:23 -05:00 committed by Junio C Hamano
parent a0ba80001a
commit ee798742bd
1 changed files with 14 additions and 5 deletions
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19
merge-recursive.c
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@ -1712,6 +1712,15 @@ static char *find_path_for_conflict(struct merge_options *opt,
return new_path;
}
/*
* Toggle the stage number between "ours" and "theirs" (2 and 3) by flipping
* the 1-bit.
*/
static inline int flip_stage(int stage)
{
return stage ^ 1;
}
static int handle_rename_rename_1to2(struct merge_options *opt,
struct rename_conflict_info *ci)
{
@ -1756,14 +1765,14 @@ static int handle_rename_rename_1to2(struct merge_options *opt,
* such cases, we should keep the added file around,
* resolving the conflict at that path in its favor.
*/
add = &ci->ren1->dst_entry->stages[2 ^ 1];
add = &ci->ren1->dst_entry->stages[flip_stage(2)];
if (is_valid(add)) {
if (update_file(opt, 0, add, a->path))
return -1;
}
else
remove_file_from_index(opt->repo->index, a->path);
add = &ci->ren2->dst_entry->stages[3 ^ 1];
add = &ci->ren2->dst_entry->stages[flip_stage(3)];
if (is_valid(add)) {
if (update_file(opt, 0, add, b->path))
return -1;
@ -1776,7 +1785,7 @@ static int handle_rename_rename_1to2(struct merge_options *opt,
* rename/add collision. If not, we can write the file out
* to the specified location.
*/
add = &ci->ren1->dst_entry->stages[2 ^ 1];
add = &ci->ren1->dst_entry->stages[flip_stage(2)];
if (is_valid(add)) {
add->path = mfi.blob.path = a->path;
if (handle_file_collision(opt, a->path,
@ -1797,7 +1806,7 @@ static int handle_rename_rename_1to2(struct merge_options *opt,
return -1;
}
add = &ci->ren2->dst_entry->stages[3 ^ 1];
add = &ci->ren2->dst_entry->stages[flip_stage(3)];
if (is_valid(add)) {
add->path = mfi.blob.path = b->path;
if (handle_file_collision(opt, b->path,
@ -1846,7 +1855,7 @@ static int handle_rename_rename_2to1(struct merge_options *opt,
path_side_1_desc = xstrfmt("version of %s from %s", path, a->path);
path_side_2_desc = xstrfmt("version of %s from %s", path, b->path);
ostage1 = ci->ren1->branch == opt->branch1 ? 3 : 2;
ostage2 = ostage1 ^ 1;
ostage2 = flip_stage(ostage1);
ci->ren1->src_entry->stages[ostage1].path = a->path;
ci->ren2->src_entry->stages[ostage2].path = b->path;
if (merge_mode_and_contents(opt, a, c1,