| Message ID | 20240809130418.10431-1-alejandro.vallejo@cloud.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC] xen: Remove -Wdeclaration-after-statement | expand |
On 09.08.2024 15:04, Alejandro Vallejo wrote: > This warning only makes sense when developing using a compiler with C99 > support on a codebase meant to be built with C89 compilers too, and > that's no longer the case (nor should it be, as it's been 25 years since > C99 came out already). > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> > --- > Yes, I'm opening this can of worms. I'd like to hear others people's > thoughts on this and whether this is something MISRA has views on. If > there's an ulterior non-obvious reason besides stylistic preference I > think it should be documented somewhere, but I haven't seen such an > explanation. > > IMO, the presence of this warning causes several undesirable effects: > > 1. Small functions are hampered by the preclusion of check+declare > patterns that improve readability via concision. e.g: Consider a > silly example like: > > /* with warning */ /* without warning */ > void foo(uint8_t *p) void foo(uint8_t *p) > { { > uint8_t tmp1; if ( !p ) > uint16_t tmp2; return; > uint32_t tmp3; > uint8_t tmp1 = OFFSET1 + *p; > if ( !p ) uint16_t tmp2 = OFFSET2 + *p; > return; uint32_t tmp3 = OFFSET3 + *p; > > tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ > tmp2 = OFFSET2 + *p; } > tmp2 = OFFSET2 + *p; > > /* Lots of uses of tmpX */ > } > > 2. Promotes scope-creep. On small functions it doesn't matter much, > but on bigger ones to prevent declaring halfway through the body > needlessly increases variable scope to the full scope in which they > are defined rather than the subscope of point-of-declaration to > end-of-current-scope. In cases in which they can be trivially > defined at that point, it also means they can be trivially misused > before they are meant to. i.e: On the example in (1) assume the > conditional in "with warning" is actually a large switch statement. > > 3. It facilitates a disconnect between time-of-declaration and > time-of-use that lead very easily to "use-before-init" bugs. > While a modern compiler can alleviate the most egregious cases of > this, there's cases it simply cannot cover. A conditional > initialization on anything with external linkage combined with a > conditional use on something else with external linkage will squash > the warning of using an uninitialised variable. Things are worse > where the variable in question is preinitialised to something > credible (e.g: a pointer to NULL), as then that can be misused > between its declaration and its original point of intended use. Right, these are the aspects that would improve. The potential downside is that you no longer have a fixed place (set of places) where to look for which variables are actually in scope. For people having worked with C89 (and not e.g. C++) for a very long time, mixing of declarations and statements may be irritating. In fact, having used C++ quite a lot in the (meanwhile distant) past, I have developed a mental C mode and a mental C++ one - when in the former I expect declarations at the start of scopes, while when in the latter I know to expect them everywhere. All in all - I'm afraid I'm split on this. Jan
Adding Roberto Does MISRA have a view on this? I seem to remember this is discouraged? On Fri, 9 Aug 2024, Alejandro Vallejo wrote: > This warning only makes sense when developing using a compiler with C99 > support on a codebase meant to be built with C89 compilers too, and > that's no longer the case (nor should it be, as it's been 25 years since > C99 came out already). > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> > --- > Yes, I'm opening this can of worms. I'd like to hear others people's > thoughts on this and whether this is something MISRA has views on. If > there's an ulterior non-obvious reason besides stylistic preference I > think it should be documented somewhere, but I haven't seen such an > explanation. > > IMO, the presence of this warning causes several undesirable effects: > > 1. Small functions are hampered by the preclusion of check+declare > patterns that improve readability via concision. e.g: Consider a > silly example like: > > /* with warning */ /* without warning */ > void foo(uint8_t *p) void foo(uint8_t *p) > { { > uint8_t tmp1; if ( !p ) > uint16_t tmp2; return; > uint32_t tmp3; > uint8_t tmp1 = OFFSET1 + *p; > if ( !p ) uint16_t tmp2 = OFFSET2 + *p; > return; uint32_t tmp3 = OFFSET3 + *p; > > tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ > tmp2 = OFFSET2 + *p; } > tmp2 = OFFSET2 + *p; > > /* Lots of uses of tmpX */ > } > > 2. Promotes scope-creep. On small functions it doesn't matter much, > but on bigger ones to prevent declaring halfway through the body > needlessly increases variable scope to the full scope in which they > are defined rather than the subscope of point-of-declaration to > end-of-current-scope. In cases in which they can be trivially > defined at that point, it also means they can be trivially misused > before they are meant to. i.e: On the example in (1) assume the > conditional in "with warning" is actually a large switch statement. > > 3. It facilitates a disconnect between time-of-declaration and > time-of-use that lead very easily to "use-before-init" bugs. > While a modern compiler can alleviate the most egregious cases of > this, there's cases it simply cannot cover. A conditional > initialization on anything with external linkage combined with a > conditional use on something else with external linkage will squash > the warning of using an uninitialised variable. Things are worse > where the variable in question is preinitialised to something > credible (e.g: a pointer to NULL), as then that can be misused > between its declaration and its original point of intended use. > > So... thoughts? yay or nay? In my opinion, there are some instances where mixing declarations and statements would enhance the code, but these are uncommon. Given the choice between: 1) declarations always first 2) declarations always mixed with statements I would choose 1). One could say that mixing declarations with statements should be done only when appropriate, but the challenge lies in the subjective nature of "when appropriate." Different individuals have varying interpretations of this, which could lead to unnecessary bikeshedding. For these reasons, I do not support mixing declarations and statements.
On Fri Aug 9, 2024 at 8:25 PM BST, Stefano Stabellini wrote: > Adding Roberto > > Does MISRA have a view on this? I seem to remember this is discouraged? > I'd be surprised if MISRA didn't promote declaring close to first use to avoid use-before-init, but you very clearly have a lot more exposure to it than I do. I'm quite curious about what its preference is and the rationale for it. > > On Fri, 9 Aug 2024, Alejandro Vallejo wrote: > > This warning only makes sense when developing using a compiler with C99 > > support on a codebase meant to be built with C89 compilers too, and > > that's no longer the case (nor should it be, as it's been 25 years since > > C99 came out already). > > > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> > > --- > > Yes, I'm opening this can of worms. I'd like to hear others people's > > thoughts on this and whether this is something MISRA has views on. If > > there's an ulterior non-obvious reason besides stylistic preference I > > think it should be documented somewhere, but I haven't seen such an > > explanation. > > > > IMO, the presence of this warning causes several undesirable effects: > > > > 1. Small functions are hampered by the preclusion of check+declare > > patterns that improve readability via concision. e.g: Consider a > > silly example like: > > > > /* with warning */ /* without warning */ > > void foo(uint8_t *p) void foo(uint8_t *p) > > { { > > uint8_t tmp1; if ( !p ) > > uint16_t tmp2; return; > > uint32_t tmp3; > > uint8_t tmp1 = OFFSET1 + *p; > > if ( !p ) uint16_t tmp2 = OFFSET2 + *p; > > return; uint32_t tmp3 = OFFSET3 + *p; > > > > tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ > > tmp2 = OFFSET2 + *p; } > > tmp2 = OFFSET2 + *p; > > > > /* Lots of uses of tmpX */ > > } > > > > 2. Promotes scope-creep. On small functions it doesn't matter much, > > but on bigger ones to prevent declaring halfway through the body > > needlessly increases variable scope to the full scope in which they > > are defined rather than the subscope of point-of-declaration to > > end-of-current-scope. In cases in which they can be trivially > > defined at that point, it also means they can be trivially misused > > before they are meant to. i.e: On the example in (1) assume the > > conditional in "with warning" is actually a large switch statement. > > > > 3. It facilitates a disconnect between time-of-declaration and > > time-of-use that lead very easily to "use-before-init" bugs. > > While a modern compiler can alleviate the most egregious cases of > > this, there's cases it simply cannot cover. A conditional > > initialization on anything with external linkage combined with a > > conditional use on something else with external linkage will squash > > the warning of using an uninitialised variable. Things are worse > > where the variable in question is preinitialised to something > > credible (e.g: a pointer to NULL), as then that can be misused > > between its declaration and its original point of intended use. > > > > So... thoughts? yay or nay? > > In my opinion, there are some instances where mixing declarations and > statements would enhance the code, but these are uncommon. Given the > choice between: > > 1) declarations always first > 2) declarations always mixed with statements > > I would choose 1). FWIW, so would I under those contraints. But let me at least try to persuade you. There's at least two more arguments to weigh: 1. It wasn't that long ago that we had to resort to GNU extensions to work around this restriction. It's mildly annoying having to play games with compiler extensions because we're purposely restricting our use of the language. See the clang codegen workaround: https://lore.kernel.org/xen-devel/D2ZM0D609TOQ.2GQQWR1QALUPA@cloud.com/ 2. There's an existing divide between toolstack and hypervisor. Toolstack already allows this kind of mixing, and it's hard not-to due to external dependencies. While style doesn't have to match 1:1, it'd be (imo) preferrable to try and remove avoidable differences. See (40be6307ec00: "Only compile the hypervisor with -Wdeclaration-after-statement") https://lore.kernel.org/xen-devel/20231205183226.26636-1-julien@xen.org/ With this in mind, ... > > One could say that mixing declarations with statements should be done > only when appropriate, but the challenge lies in the subjective nature > of "when appropriate." Different individuals have varying > interpretations of this, which could lead to unnecessary bikeshedding. ... I do agree that whatever changes we introduce, considering the usual complaints about the review process, should be in a direction of measurable objectivity so as to minimize unproductive nitpicking. In that sense, a differently worded choice is: 1. Declarations always at the beginning of the scope closest to first use. 2. Declarations always closest to first use. They really aren't that different, and we do already make reviews asking for declarations to be moved to the closest scope. Given this choice I definitely prefer (2), in part because it removes the uncertainty from review that true freeform declarations would allow, better aligns our dialect of C with the current standard and aligns the hypervisor style (slightly) with the tools it requires. Plus the advantages I already mentioned in the original email. > > For these reasons, I do not support mixing declarations and statements. Pending whatever MISRA has to say on the matter, I hope these arguments can steer your view. Cheers, Alejandrq
On 09/08/24 21:25, Stefano Stabellini wrote: > Adding Roberto > > Does MISRA have a view on this? I seem to remember this is discouraged? As far as I know, there is nothing in MISRA C against or in favor of mixing declaration with statements. The only (slightly) relevant guideline is advisory Rule 8.9 (An object should be declared at block scope if its identifier only appears in a single function), which advises to declare objects at function scope when this is possible. The rationale of the same rule says, among other things: Within a function, whether objects are declared at the outermost or innermost block is largely a matter of style. On the other hand, MISRA C recognizes that reduction of scope is, all other things being equal, to be preferred, but there are no guidelines similar to -Wdeclaration-after-statement. Rather, the point is "declare it at block scope, if you can; otherwise declare it at file scope, if you can; otherwise, declare it at global scope, if you must." Kind regards, Roberto > On Fri, 9 Aug 2024, Alejandro Vallejo wrote: >> This warning only makes sense when developing using a compiler with C99 >> support on a codebase meant to be built with C89 compilers too, and >> that's no longer the case (nor should it be, as it's been 25 years since >> C99 came out already). >> >> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> >> --- >> Yes, I'm opening this can of worms. I'd like to hear others people's >> thoughts on this and whether this is something MISRA has views on. If >> there's an ulterior non-obvious reason besides stylistic preference I >> think it should be documented somewhere, but I haven't seen such an >> explanation. >> >> IMO, the presence of this warning causes several undesirable effects: >> >> 1. Small functions are hampered by the preclusion of check+declare >> patterns that improve readability via concision. e.g: Consider a >> silly example like: >> >> /* with warning */ /* without warning */ >> void foo(uint8_t *p) void foo(uint8_t *p) >> { { >> uint8_t tmp1; if ( !p ) >> uint16_t tmp2; return; >> uint32_t tmp3; >> uint8_t tmp1 = OFFSET1 + *p; >> if ( !p ) uint16_t tmp2 = OFFSET2 + *p; >> return; uint32_t tmp3 = OFFSET3 + *p; >> >> tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ >> tmp2 = OFFSET2 + *p; } >> tmp2 = OFFSET2 + *p; >> >> /* Lots of uses of tmpX */ >> } >> >> 2. Promotes scope-creep. On small functions it doesn't matter much, >> but on bigger ones to prevent declaring halfway through the body >> needlessly increases variable scope to the full scope in which they >> are defined rather than the subscope of point-of-declaration to >> end-of-current-scope. In cases in which they can be trivially >> defined at that point, it also means they can be trivially misused >> before they are meant to. i.e: On the example in (1) assume the >> conditional in "with warning" is actually a large switch statement. >> >> 3. It facilitates a disconnect between time-of-declaration and >> time-of-use that lead very easily to "use-before-init" bugs. >> While a modern compiler can alleviate the most egregious cases of >> this, there's cases it simply cannot cover. A conditional >> initialization on anything with external linkage combined with a >> conditional use on something else with external linkage will squash >> the warning of using an uninitialised variable. Things are worse >> where the variable in question is preinitialised to something >> credible (e.g: a pointer to NULL), as then that can be misused >> between its declaration and its original point of intended use. >> >> So... thoughts? yay or nay? > > In my opinion, there are some instances where mixing declarations and > statements would enhance the code, but these are uncommon. Given the > choice between: > > 1) declarations always first > 2) declarations always mixed with statements > > I would choose 1). > > One could say that mixing declarations with statements should be done > only when appropriate, but the challenge lies in the subjective nature > of "when appropriate." Different individuals have varying > interpretations of this, which could lead to unnecessary bikeshedding. > > For these reasons, I do not support mixing declarations and statements. >
On Fri, Aug 9, 2024 at 2:04 PM Alejandro Vallejo <alejandro.vallejo@cloud.com> wrote: > > This warning only makes sense when developing using a compiler with C99 > support on a codebase meant to be built with C89 compilers too, and > that's no longer the case (nor should it be, as it's been 25 years since > C99 came out already). > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> > --- > Yes, I'm opening this can of worms. I'd like to hear others people's > thoughts on this and whether this is something MISRA has views on. If > there's an ulterior non-obvious reason besides stylistic preference I > think it should be documented somewhere, but I haven't seen such an > explanation. > > IMO, the presence of this warning causes several undesirable effects: > > 1. Small functions are hampered by the preclusion of check+declare > patterns that improve readability via concision. e.g: Consider a > silly example like: > > /* with warning */ /* without warning */ > void foo(uint8_t *p) void foo(uint8_t *p) > { { > uint8_t tmp1; if ( !p ) > uint16_t tmp2; return; > uint32_t tmp3; > uint8_t tmp1 = OFFSET1 + *p; > if ( !p ) uint16_t tmp2 = OFFSET2 + *p; > return; uint32_t tmp3 = OFFSET3 + *p; > > tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ > tmp2 = OFFSET2 + *p; } > tmp2 = OFFSET2 + *p; > > /* Lots of uses of tmpX */ > } > > 2. Promotes scope-creep. On small functions it doesn't matter much, > but on bigger ones to prevent declaring halfway through the body > needlessly increases variable scope to the full scope in which they > are defined rather than the subscope of point-of-declaration to > end-of-current-scope. In cases in which they can be trivially > defined at that point, it also means they can be trivially misused > before they are meant to. i.e: On the example in (1) assume the > conditional in "with warning" is actually a large switch statement. > > 3. It facilitates a disconnect between time-of-declaration and > time-of-use that lead very easily to "use-before-init" bugs. > While a modern compiler can alleviate the most egregious cases of > this, there's cases it simply cannot cover. A conditional > initialization on anything with external linkage combined with a > conditional use on something else with external linkage will squash > the warning of using an uninitialised variable. Things are worse > where the variable in question is preinitialised to something > credible (e.g: a pointer to NULL), as then that can be misused > between its declaration and its original point of intended use. > > So... thoughts? yay or nay? Hi, I personally agree with this change. Even compiler is stating that this is just C89 compatibility. Yes, it's question of style and surely having all declaration at the beginning reduce the options however there are technical reason why having mixed declarations can help - you can use "const" to tell compiler (and code editor) that something should not be changed; - in many cases reduces commit changes; > --- > xen/Makefile | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/xen/Makefile b/xen/Makefile > index 2e1a925c8417..288b7ac8bb2d 100644 > --- a/xen/Makefile > +++ b/xen/Makefile > @@ -394,7 +394,7 @@ CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections > > CFLAGS += -nostdinc -fno-builtin -fno-common > CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith > -CFLAGS += -Wdeclaration-after-statement -Wuninitialized > +CFLAGS += -Wuninitialized > $(call cc-option-add,CFLAGS,CC,-Wvla) > $(call cc-option-add,CFLAGS,CC,-Wflex-array-member-not-at-end) > $(call cc-option-add,CFLAGS,CC,-Winit-self) Frediano
diff --git a/xen/Makefile b/xen/Makefile index 2e1a925c8417..288b7ac8bb2d 100644 --- a/xen/Makefile +++ b/xen/Makefile @@ -394,7 +394,7 @@ CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections CFLAGS += -nostdinc -fno-builtin -fno-common CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith -CFLAGS += -Wdeclaration-after-statement -Wuninitialized +CFLAGS += -Wuninitialized $(call cc-option-add,CFLAGS,CC,-Wvla) $(call cc-option-add,CFLAGS,CC,-Wflex-array-member-not-at-end) $(call cc-option-add,CFLAGS,CC,-Winit-self)
This warning only makes sense when developing using a compiler with C99 support on a codebase meant to be built with C89 compilers too, and that's no longer the case (nor should it be, as it's been 25 years since C99 came out already). Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com> --- Yes, I'm opening this can of worms. I'd like to hear others people's thoughts on this and whether this is something MISRA has views on. If there's an ulterior non-obvious reason besides stylistic preference I think it should be documented somewhere, but I haven't seen such an explanation. IMO, the presence of this warning causes several undesirable effects: 1. Small functions are hampered by the preclusion of check+declare patterns that improve readability via concision. e.g: Consider a silly example like: /* with warning */ /* without warning */ void foo(uint8_t *p) void foo(uint8_t *p) { { uint8_t tmp1; if ( !p ) uint16_t tmp2; return; uint32_t tmp3; uint8_t tmp1 = OFFSET1 + *p; if ( !p ) uint16_t tmp2 = OFFSET2 + *p; return; uint32_t tmp3 = OFFSET3 + *p; tmp1 = OFFSET1 + *p; /* Lots of uses of `tmpX` */ tmp2 = OFFSET2 + *p; } tmp2 = OFFSET2 + *p; /* Lots of uses of tmpX */ } 2. Promotes scope-creep. On small functions it doesn't matter much, but on bigger ones to prevent declaring halfway through the body needlessly increases variable scope to the full scope in which they are defined rather than the subscope of point-of-declaration to end-of-current-scope. In cases in which they can be trivially defined at that point, it also means they can be trivially misused before they are meant to. i.e: On the example in (1) assume the conditional in "with warning" is actually a large switch statement. 3. It facilitates a disconnect between time-of-declaration and time-of-use that lead very easily to "use-before-init" bugs. While a modern compiler can alleviate the most egregious cases of this, there's cases it simply cannot cover. A conditional initialization on anything with external linkage combined with a conditional use on something else with external linkage will squash the warning of using an uninitialised variable. Things are worse where the variable in question is preinitialised to something credible (e.g: a pointer to NULL), as then that can be misused between its declaration and its original point of intended use. So... thoughts? yay or nay? --- xen/Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)