Subject: Re: [PATCH v3 00/10] Avoid the comma operator

From: Phillip Wood <[email-masked]>
Date: Thu, 27 Mar 2025 15:07:17 +0000

Hi Johannes

On 27/03/2025 11:52, Johannes Schindelin via GitGitGadget wrote:
> 
> Changes since v2:
> 
>   * Made the sed construct in detect-compiler portable (thanks, Eric
>     Sunshine!)
>   * The majority of the feedback disagreed with the more compact format in
>     diff-delta.c, so I changed it to the long format (thanks, Phillip Wood!)
>   * The more succinct and safer, but less readable, cast in the loop
>     condition of the dowild() function was replaced with the goto-based
>     alternative I had mentioned as a possibility in the commit message
>     (thanks, Phillip Wood!)
>   * I adjusted the style of my compat/regex/ patch to the surrounding code's.
>   * The -Wcomma option is now used in Meson-based clang builds, too (thanks,
>     Patrick Steinhardt!)

The range-diff below looks good to me, thanks for making our code base 
clearer.

Best Wishes

Phillip

> Range-diff vs v2:
> 
>    1:  913c7a0d296 =  1:  913c7a0d296 remote-curl: avoid using the comma operator unnecessarily
>    2:  37ff88b8275 =  2:  37ff88b8275 rebase: avoid using the comma operator unnecessarily
>    3:  f601f4e74a5 =  3:  f601f4e74a5 kwset: avoid using the comma operator unnecessarily
>    4:  f60ebe376e1 =  4:  f60ebe376e1 clar: avoid using the comma operator unnecessarily
>    5:  7239078413f =  5:  7239078413f xdiff: avoid using the comma operator unnecessarily
>    6:  5e0e8325620 !  6:  045d695d73e diff-delta: explicitly mark intentional use of the comma operator
>       @@ Metadata
>        Author: Johannes Schindelin <[email-masked]>
>        
>         ## Commit message ##
>       -    diff-delta: explicitly mark intentional use of the comma operator
>       +    diff-delta: avoid using the comma operator
>        
>            The comma operator is a somewhat obscure C feature that is often used by
>            mistake and can even cause unintentional code flow. That is why the
>       @@ Commit message
>            Intentional uses include situations where one wants to avoid curly
>            brackets around multiple statements that need to be guarded by a
>            condition. This is the case here, as the repetitive nature of the
>       -    statements is easier to see for a human reader this way.
>       +    statements is easier to see for a human reader this way. At least in my
>       +    opinion.
>        
>       -    To mark this usage as intentional, the return value of the statement
>       -    before the comma needs to be cast to `void`, which we do here.
>       +    However, opinions on this differ wildly, take 10 people and you have 10
>       +    different preferences.
>        
>       +    On the Git mailing list, it seems that the consensus is to use the long
>       +    form instead, so let's do just that.
>       +
>       +    Suggested-by: Phillip Wood <[email-masked]>
>            Signed-off-by: Johannes Schindelin <[email-masked]>
>        
>         ## diff-delta.c ##
>        @@ diff-delta.c: create_delta(const struct delta_index *index,
>       + 			op = out + outpos++;
>         			i = 0x80;
>         
>       - 			if (moff & 0x000000ff)
>       +-			if (moff & 0x000000ff)
>        -				out[outpos++] = moff >> 0,  i |= 0x01;
>       -+				(void)(out[outpos++] = moff >> 0),  i |= 0x01;
>       - 			if (moff & 0x0000ff00)
>       +-			if (moff & 0x0000ff00)
>        -				out[outpos++] = moff >> 8,  i |= 0x02;
>       -+				(void)(out[outpos++] = moff >> 8),  i |= 0x02;
>       - 			if (moff & 0x00ff0000)
>       +-			if (moff & 0x00ff0000)
>        -				out[outpos++] = moff >> 16, i |= 0x04;
>       -+				(void)(out[outpos++] = moff >> 16), i |= 0x04;
>       - 			if (moff & 0xff000000)
>       +-			if (moff & 0xff000000)
>        -				out[outpos++] = moff >> 24, i |= 0x08;
>       -+				(void)(out[outpos++] = moff >> 24), i |= 0x08;
>       -
>       - 			if (msize & 0x00ff)
>       +-
>       +-			if (msize & 0x00ff)
>        -				out[outpos++] = msize >> 0, i |= 0x10;
>       -+				(void)(out[outpos++] = msize >> 0), i |= 0x10;
>       - 			if (msize & 0xff00)
>       +-			if (msize & 0xff00)
>        -				out[outpos++] = msize >> 8, i |= 0x20;
>       -+				(void)(out[outpos++] = msize >> 8), i |= 0x20;
>       ++			if (moff & 0x000000ff) {
>       ++				out[outpos++] = moff >> 0;
>       ++				i |= 0x01;
>       ++			}
>       ++			if (moff & 0x0000ff00) {
>       ++				out[outpos++] = moff >> 8;
>       ++				i |= 0x02;
>       ++			}
>       ++			if (moff & 0x00ff0000) {
>       ++				out[outpos++] = moff >> 16;
>       ++				i |= 0x04;
>       ++			}
>       ++			if (moff & 0xff000000) {
>       ++				out[outpos++] = moff >> 24;
>       ++				i |= 0x08;
>       ++			}
>       ++
>       ++			if (msize & 0x00ff) {
>       ++				out[outpos++] = msize >> 0;
>       ++				i |= 0x10;
>       ++			}
>       ++			if (msize & 0xff00) {
>       ++				out[outpos++] = msize >> 8;
>       ++				i |= 0x20;
>       ++			}
>         
>         			*op = i;
>         
>    7:  9a6de12b807 !  7:  1d0ce59cb68 wildmatch: explicitly mark intentional use of the comma operator
>       @@ Metadata
>        Author: Johannes Schindelin <[email-masked]>
>        
>         ## Commit message ##
>       -    wildmatch: explicitly mark intentional use of the comma operator
>       +    wildmatch: avoid using of the comma operator
>        
>            The comma operator is a somewhat obscure C feature that is often used by
>            mistake and can even cause unintentional code flow. That is why the
>            `-Wcomma` option of clang was introduced: To identify unintentional uses
>            of the comma operator.
>        
>       -    To mark such a usage as intentional, the value needs to be cast to
>       -    `void`, which we do here.
>       -
>            In this instance, the usage is intentional because it allows storing the
>            value of the current character as `prev_ch` before making the next
>            character the current one, all of which happens in the loop condition
>            that lets the loop stop at a closing bracket.
>        
>       -    The alternative to using the comma operator would be to move those
>       +    However, it is hard to read.
>       +
>       +    The chosen alternative to using the comma operator is to move those
>            assignments from the condition into the loop body; In this particular
>       -    case that would require the assignments to either be duplicated or to
>       -    introduce and use a `goto` target before the assignments, though,
>       -    because the loop body contains a `continue` for the case where a
>       -    character class is found that starts with `[:` but does not end in `:]`
>       -    (and the assignments should occur even when that code path is taken).
>       +    case that requires special care because the loop body contains a
>       +    `continue` for the case where a character class is found that starts
>       +    with `[:` but does not end in `:]` (and the assignments should occur
>       +    even when that code path is taken), which needs to be turned into a
>       +    `goto`.
>        
>       +    Helped-by: Phillip Wood <[email-masked]>
>            Signed-off-by: Johannes Schindelin <[email-masked]>
>        
>         ## wildmatch.c ##
>       +@@ wildmatch.c: static int dowild(const uchar *p, const uchar *text, unsigned int flags)
>       + 						p_ch = '[';
>       + 						if (t_ch == p_ch)
>       + 							matched = 1;
>       +-						continue;
>       ++						goto next;
>       + 					}
>       + 					if (CC_EQ(s,i, "alnum")) {
>       + 						if (ISALNUM(t_ch))
>        @@ wildmatch.c: static int dowild(const uchar *p, const uchar *text, unsigned int flags)
>         					p_ch = 0; /* This makes "prev_ch" get set to 0. */
>         				} else if (t_ch == p_ch)
>         					matched = 1;
>        -			} while (prev_ch = p_ch, (p_ch = *++p) != ']');
>       -+			} while ((void)(prev_ch = p_ch), (p_ch = *++p) != ']');
>       ++next:
>       ++				prev_ch = p_ch;
>       ++				p_ch = *++p;
>       ++			} while (p_ch != ']');
>         			if (matched == negated ||
>         			    ((flags & WM_PATHNAME) && t_ch == '/'))
>         				return WM_NOMATCH;
>    8:  dc626f36df3 !  8:  b8405f3d237 compat/regex: explicitly mark intentional use of the comma operator
>       @@ Commit message
>        
>         ## compat/regex/regex_internal.c ##
>        @@ compat/regex/regex_internal.c: re_node_set_merge (re_node_set *dest, const re_node_set *src)
>       -   for (sbase = dest->nelem + 2 * src->nelem,
>                is = src->nelem - 1, id = dest->nelem - 1; is >= 0 && id >= 0; )
>             {
>       --      if (dest->elems[id] == src->elems[is])
>       +       if (dest->elems[id] == src->elems[is])
>        -	is--, id--;
>       --      else if (dest->elems[id] < src->elems[is])
>       -+      if (dest->elems[id] == src->elems[is]) {
>       -+	is--;
>       -+	id--;
>       -+      } else if (dest->elems[id] < src->elems[is])
>       ++	{
>       ++	  is--;
>       ++	  id--;
>       ++	}
>       +       else if (dest->elems[id] < src->elems[is])
>         	dest->elems[--sbase] = src->elems[is--];
>               else /* if (dest->elems[id] > src->elems[is]) */
>       - 	--id;
>        
>         ## compat/regex/regexec.c ##
>        @@ compat/regex/regexec.c: sift_states_bkref (const re_match_context_t *mctx, re_sift_context_t *sctx,
>    9:  91f86c3aba9 !  9:  6b6cd556465 clang: warn when the comma operator is used
>       @@ Commit message
>            warn about code using the comma operator (because it is typically
>            unintentional and wants to use the semicolon instead).
>        
>       +    Helped-by: Patrick Steinhardt <[email-masked]>
>            Signed-off-by: Johannes Schindelin <[email-masked]>
>        
>         ## config.mak.dev ##
>       @@ config.mak.dev: DEVELOPER_CFLAGS += -Wvla
>         ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
>         DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare
>         endif
>       +
>       + ## meson.build ##
>       +@@ meson.build: libgit_dependencies = [ ]
>       + # Makefile.
>       + if get_option('warning_level') in ['2','3', 'everything'] and compiler.get_argument_syntax() == 'gcc'
>       +   foreach cflag : [
>       ++    '-Wcomma',
>       +     '-Wdeclaration-after-statement',
>       +     '-Wformat-security',
>       +     '-Wold-style-definition',
>   10:  2f6f31240fe ! 10:  77f1dcaca1c detect-compiler: detect clang even if it found CUDA
>       @@ Commit message
>            Let's unconfuse the script by letting it parse the first matching line
>            and ignore the rest.
>        
>       +    Helped-by: Eric Sunshine <[email-masked]>
>            Signed-off-by: Johannes Schindelin <[email-masked]>
>        
>         ## detect-compiler ##
>       @@ detect-compiler: CC="$*"
>         # FreeBSD clang version 3.4.1 (tags/RELEASE...)
>         get_version_line() {
>        -	LANG=C LC_ALL=C $CC -v 2>&1 | grep ' version '
>       -+	LANG=C LC_ALL=C $CC -v 2>&1 | sed -n '/ version /{p;q}'
>       ++	LANG=C LC_ALL=C $CC -v 2>&1 | sed -n '/ version /{p;q;}'
>         }
>         
>         get_family() {
>