| Message ID | 466b4c6d-032f-fbcc-58ac-75f6f39d734f@web.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mmc: atmel-mci: Reduce scope for the variable “slot” in atmci_request_end() | expand |
Hello, On 10/12/2020 16:01:44+0100, Markus Elfring wrote: > From: Markus Elfring <elfring@users.sourceforge.net> > Date: Thu, 10 Dec 2020 15:56:13 +0100 > > A local variable was used only within an if branch. > Thus move the definition for the variable “slot” into the corresponding > code block. > What is the improvement here? This makes the code harder to read. > This issue was detected by using the Coccinelle software. > > Signed-off-by: Markus Elfring <elfring@users.sourceforge.net> > --- > drivers/mmc/host/atmel-mci.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/drivers/mmc/host/atmel-mci.c b/drivers/mmc/host/atmel-mci.c > index 444bd3a0a922..6a0d999ee82e 100644 > --- a/drivers/mmc/host/atmel-mci.c > +++ b/drivers/mmc/host/atmel-mci.c > @@ -1558,7 +1558,6 @@ static void atmci_request_end(struct atmel_mci *host, struct mmc_request *mrq) > __releases(&host->lock) > __acquires(&host->lock) > { > - struct atmel_mci_slot *slot = NULL; > struct mmc_host *prev_mmc = host->cur_slot->mmc; > > WARN_ON(host->cmd || host->data); > @@ -1579,8 +1578,9 @@ static void atmci_request_end(struct atmel_mci *host, struct mmc_request *mrq) > host->cur_slot->mrq = NULL; > host->mrq = NULL; > if (!list_empty(&host->queue)) { > - slot = list_entry(host->queue.next, > - struct atmel_mci_slot, queue_node); > + struct atmel_mci_slot *slot = list_entry(host->queue.next, > + struct atmel_mci_slot, > + queue_node); > list_del(&slot->queue_node); > dev_vdbg(&host->pdev->dev, "list not empty: %s is next\n", > mmc_hostname(slot->mmc)); > -- > 2.29.2 >
>> A local variable was used only within an if branch. >> Thus move the definition for the variable “slot” into the corresponding >> code block. >> > > What is the improvement here? A possible refactoring. https://refactoring.com/catalog/reduceScopeOfVariable.html > This makes the code harder to read. Can the extra null pointer initialisation trigger a source code analysis warning like “Addresses-Coverity: ("Unused value")” for this function implementation? Regards, Markus
On 10/12/2020 17:35:31+0100, Markus Elfring wrote: > >> A local variable was used only within an if branch. > >> Thus move the definition for the variable “slot” into the corresponding > >> code block. > >> > > > > What is the improvement here? > > A possible refactoring. > https://refactoring.com/catalog/reduceScopeOfVariable.html > I quote: "Since declarations of variables in many cases costs computational cycles, you may end up wasting time for nothing." This is false, it doesn't. I also quote: "When I'm writing new code I find I don't scope my temps any less than method scope. This is because I keep my methods short, so reducing scope any further doesn't add much value. The value of this refactoring is in breaking up a large method." Is that function large? It is not. > > > This makes the code harder to read. > > Can the extra null pointer initialisation trigger a source code analysis warning > like “Addresses-Coverity: ("Unused value")” for this function implementation? > Did you check, does it? It doesn't. Are you wasting maintainer and reviewer's time? Yes you are.
>> Can the extra null pointer initialisation trigger a source code analysis warning >> like “Addresses-Coverity: ("Unused value")” for this function implementation? >> > > Did you check, does it? It doesn't. > > Are you wasting maintainer and reviewer's time? Yes you are. How do you think about a patch like “staging: speakup: remove redundant initialization of pointer p_key” for comparison? https://lore.kernel.org/patchwork/patch/1199128/ https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ Would you tolerate to omit the initialisation for the variable “slot”? Regards, Markus
On 10/12/2020 18:23:05+0100, Markus Elfring wrote: > >> Can the extra null pointer initialisation trigger a source code analysis warning > >> like “Addresses-Coverity: ("Unused value")” for this function implementation? > >> > > > > Did you check, does it? It doesn't. > > > > Are you wasting maintainer and reviewer's time? Yes you are. > > How do you think about a patch like “staging: speakup: remove redundant initialization > of pointer p_key” for comparison? > https://lore.kernel.org/patchwork/patch/1199128/ > https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ > > Would you tolerate to omit the initialisation for the variable “slot”? > If you were able to provide one good technical reason.
>> How do you think about a patch like “staging: speakup: remove redundant initialization >> of pointer p_key” for comparison? >> https://lore.kernel.org/patchwork/patch/1199128/ >> https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ >> >> Would you tolerate to omit the initialisation for the variable “slot”? > > If you were able to provide one good technical reason. I find that the positions of variable definitions (and similar assignments) influence the generation of executable code. How do you think about another bit of background information? Example: [PATCH v3 2/3] gianfar: Delete unnecessary variable initialisations in gfar_ethflow_to_filer_table() https://lore.kernel.org/patchwork/patch/635859/#821314 https://lore.kernel.org/lkml/20160115121510.GJ4764@mwanda/ Regards, Markus
On 11/12/2020 07:34:41+0100, Markus Elfring wrote: > >> How do you think about a patch like “staging: speakup: remove redundant initialization > >> of pointer p_key” for comparison? > >> https://lore.kernel.org/patchwork/patch/1199128/ > >> https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ > >> > >> Would you tolerate to omit the initialisation for the variable “slot”? > > > > If you were able to provide one good technical reason. > > I find that the positions of variable definitions (and similar assignments) influence > the generation of executable code. > And you are wrong, it doesn't. Before: c044a0f0 <atmci_request_end>: { c044a0f0: e92d4070 push {r4, r5, r6, lr} c044a0f4: e1a04000 mov r4, r0 WARN_ON(host->cmd || host->data); c044a0f8: e5902024 ldr r2, [r0, #36] ; 0x24 { c044a0fc: e1a06001 mov r6, r1 struct mmc_host *prev_mmc = host->cur_slot->mmc; c044a100: e590301c ldr r3, [r0, #28] WARN_ON(host->cmd || host->data); c044a104: e3520000 cmp r2, #0 struct mmc_host *prev_mmc = host->cur_slot->mmc; c044a108: e5935000 ldr r5, [r3] WARN_ON(host->cmd || host->data); c044a10c: 0a00002d beq c044a1c8 <atmci_request_end+0xd8> c044a110: e3000000 movw r0, #0 c044a110: R_ARM_MOVW_ABS_NC .LC0 c044a114: e3a03000 mov r3, #0 c044a118: e3400000 movt r0, #0 c044a118: R_ARM_MOVT_ABS .LC0 c044a11c: e3a02009 mov r2, #9 c044a120: e300161c movw r1, #1564 ; 0x61c c044a124: ebfffffe bl 0 <warn_slowpath_fmt> c044a124: R_ARM_CALL warn_slowpath_fmt del_timer(&host->timer); c044a128: e28400a4 add r0, r4, #164 ; 0xa4 c044a12c: ebfffffe bl 0 <del_timer> c044a12c: R_ARM_CALL del_timer if (host->need_clock_update) { c044a130: e5d430a0 ldrb r3, [r4, #160] ; 0xa0 c044a134: e3530000 cmp r3, #0 c044a138: 0a000005 beq c044a154 <atmci_request_end+0x64> atmci_writel(host, ATMCI_MR, host->mode_reg); c044a13c: e59420b8 ldr r2, [r4, #184] ; 0xb8 c044a140: e5943000 ldr r3, [r4] asm volatile("str %1, %0" c044a144: e5832004 str r2, [r3, #4] if (host->caps.has_cfg_reg) c044a148: e5d420da ldrb r2, [r4, #218] ; 0xda c044a14c: e3520000 cmp r2, #0 c044a150: 1a000019 bne c044a1bc <atmci_request_end+0xcc> host->cur_slot->mrq = NULL; c044a154: e594101c ldr r1, [r4, #28] return READ_ONCE(head->next) == head; c044a158: e1a03004 mov r3, r4 c044a15c: e3a02000 mov r2, #0 c044a160: e5812010 str r2, [r1, #16] host->mrq = NULL; c044a164: e5842020 str r2, [r4, #32] c044a168: e5b31098 ldr r1, [r3, #152]! ; 0x98 if (!list_empty(&host->queue)) { c044a16c: e1510003 cmp r1, r3 host->state = STATE_IDLE; c044a170: 05842094 streq r2, [r4, #148] ; 0x94 if (!list_empty(&host->queue)) { c044a174: 0a00000c beq c044a1ac <atmci_request_end+0xbc> slot = list_entry(host->queue.next, c044a178: e5943098 ldr r3, [r4, #152] ; 0x98 After: c044a0f0 <atmci_request_end>: { c044a0f0: e92d4070 push {r4, r5, r6, lr} c044a0f4: e1a04000 mov r4, r0 WARN_ON(host->cmd || host->data); c044a0f8: e5902024 ldr r2, [r0, #36] ; 0x24 { c044a0fc: e1a06001 mov r6, r1 struct mmc_host *prev_mmc = host->cur_slot->mmc; c044a100: e590301c ldr r3, [r0, #28] WARN_ON(host->cmd || host->data); c044a104: e3520000 cmp r2, #0 struct mmc_host *prev_mmc = host->cur_slot->mmc; c044a108: e5935000 ldr r5, [r3] WARN_ON(host->cmd || host->data); c044a10c: 0a00002d beq c044a1c8 <atmci_request_end+0xd8> c044a110: e3000000 movw r0, #0 c044a110: R_ARM_MOVW_ABS_NC .LC0 c044a114: e3a03000 mov r3, #0 c044a118: e3400000 movt r0, #0 c044a118: R_ARM_MOVT_ABS .LC0 c044a11c: e3a02009 mov r2, #9 c044a120: e300161b movw r1, #1563 ; 0x61b c044a124: ebfffffe bl 0 <warn_slowpath_fmt> c044a124: R_ARM_CALL warn_slowpath_fmt del_timer(&host->timer); c044a128: e28400a4 add r0, r4, #164 ; 0xa4 c044a12c: ebfffffe bl 0 <del_timer> c044a12c: R_ARM_CALL del_timer if (host->need_clock_update) { c044a130: e5d430a0 ldrb r3, [r4, #160] ; 0xa0 c044a134: e3530000 cmp r3, #0 c044a138: 0a000005 beq c044a154 <atmci_request_end+0x64> atmci_writel(host, ATMCI_MR, host->mode_reg); c044a13c: e59420b8 ldr r2, [r4, #184] ; 0xb8 c044a140: e5943000 ldr r3, [r4] asm volatile("str %1, %0" c044a144: e5832004 str r2, [r3, #4] if (host->caps.has_cfg_reg) c044a148: e5d420da ldrb r2, [r4, #218] ; 0xda c044a14c: e3520000 cmp r2, #0 c044a150: 1a000019 bne c044a1bc <atmci_request_end+0xcc> host->cur_slot->mrq = NULL; c044a154: e594101c ldr r1, [r4, #28] return READ_ONCE(head->next) == head; c044a158: e1a03004 mov r3, r4 c044a15c: e3a02000 mov r2, #0 c044a160: e5812010 str r2, [r1, #16] host->mrq = NULL; c044a164: e5842020 str r2, [r4, #32] c044a168: e5b31098 ldr r1, [r3, #152]! ; 0x98 if (!list_empty(&host->queue)) { c044a16c: e1510003 cmp r1, r3 host->state = STATE_IDLE; c044a170: 05842094 streq r2, [r4, #148] ; 0x94 if (!list_empty(&host->queue)) { c044a174: 0a00000c beq c044a1ac <atmci_request_end+0xbc> struct atmel_mci_slot *slot = list_entry(host->queue.next, c044a178: e5943098 ldr r3, [r4, #152] ; 0x98 Do you realize your patch is just unnecessary churn now? Is it too difficult for you to actually compile the driver and look at the changes before submitting patches?
Markus was banned from vger at the end of July after ignoring repeated warnings. This makes it hard to review any patches or follow discussion... regards, dan carpenter
> Markus was banned from vger at the end of July after ignoring repeated warnings. There were special disagreements and communication difficulties involved. > This makes it hard to review any patches or follow discussion... You shared also special software development opinions about extra variable initialisations occasionally, didn't you? Regards, Markus
On Fri, 2020-12-11 at 09:03 +0100, Alexandre Belloni wrote: > On 11/12/2020 07:34:41+0100, Markus Elfring wrote: > > > > How do you think about a patch like “staging: speakup: remove redundant initialization > > > > of pointer p_key” for comparison? > > > > https://lore.kernel.org/patchwork/patch/1199128/ > > > > https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ > > > > > > > > Would you tolerate to omit the initialisation for the variable “slot”? > > > > > > If you were able to provide one good technical reason. > > > > I find that the positions of variable definitions (and similar assignments) influence > > the generation of executable code. > > > And you are wrong, it doesn't. I rarely reply or read any Markus' emails as everything from Markus goes into a 'don't read' folder but I was cc'd directly on one from someone else recently so I think I should reply to this one too. In this case Alexandre it seems true, but in the generic case it may be false. It may depend on stack size and location. For instance, with large structs declared either at the top of a function or in separate branches within the function: $ cat t_structs.c struct a { int a[2000]; int b[4000]; }; struct b { char a[100]; char b[10000]; }; void foo1(struct a *a); void foo2(struct b *b); void foo(int index) { if (index) { struct a ai = {}; ai.a[index] = index; foo1(&ai); } else { struct b bi = {}; bi.b[0] = 1; foo2(&bi); } } void bar(int index) { struct a ai = {}; struct b bi = {}; if (index) { ai.a[index] = index; foo1(&ai); } else { bi.b[0] = 1; foo2(&bi); } } $ newer gcc versions are smart enough to minimize stack use in foo() but not bar() so ai and bi start at the same address in foo() so the total stack used is smaller. older gcc versions like 4.8 use separate addresses for ai and bi in foo() so the total stack used is larger. $ gcc-4.8 -Wframe-larger-than=1000 -c t_structs.c t_structs.c: In function ‘foo’: t_structs.c:27:1: warning: the frame size of 34116 bytes is larger than 1000 bytes [-Wframe-larger-than=] } ^ t_structs.c: In function ‘bar’: t_structs.c:41:1: warning: the frame size of 34116 bytes is larger than 1000 bytes [-Wframe-larger-than=] } ^ $ gcc-5 -Wframe-larger-than=1000 -c t_structs.c t_structs.c: In function ‘foo’: t_structs.c:27:1: warning: the frame size of 24032 bytes is larger than 1000 bytes [-Wframe-larger-than=] } ^ t_structs.c: In function ‘bar’: t_structs.c:41:1: warning: the frame size of 34144 bytes is larger than 1000 bytes [-Wframe-larger-than=] } ^
> In this case Alexandre it seems true, but in the generic case > it may be false. It may depend on stack size and location. Thanks for your constructive feedback. Will any other source code transformations become more interesting for the usage of bigger arrays instead of the proposal to avoid extra null pointer initialisations by adjusting the position for the desired variable definition? Regards, Markus
On 12/12/2020 01:16:39-0800, Joe Perches wrote: > On Fri, 2020-12-11 at 09:03 +0100, Alexandre Belloni wrote: > > On 11/12/2020 07:34:41+0100, Markus Elfring wrote: > > > > > How do you think about a patch like “staging: speakup: remove redundant initialization > > > > > of pointer p_key” for comparison? > > > > > https://lore.kernel.org/patchwork/patch/1199128/ > > > > > https://lore.kernel.org/driverdev-devel/20200223153954.420731-1-colin.king@canonical.com/ > > > > > > > > > > Would you tolerate to omit the initialisation for the variable “slot”? > > > > > > > > If you were able to provide one good technical reason. > > > > > > I find that the positions of variable definitions (and similar assignments) influence > > > the generation of executable code. > > > > > And you are wrong, it doesn't. > > I rarely reply or read any Markus' emails as everything > from Markus goes into a 'don't read' folder but I was cc'd > directly on one from someone else recently so I think I > should reply to this one too. > > In this case Alexandre it seems true, but in the generic case > it may be false. It may depend on stack size and location. > > For instance, with large structs declared either at the top > of a function or in separate branches within the function: > I think you realize your example is way more complicated than initializing basic type variable to 0 or NULL which has probably been optimized forever. This patch is just unnecessary churn that was generated without giving any thought. It was easy to check whether the patch actually improved anything. It doesn't and makes readability worse.
On Sat, 2020-12-12 at 14:17 +0100, Alexandre Belloni wrote: > On 12/12/2020 01:16:39-0800, Joe Perches wrote: > > In this case Alexandre it seems true, but in the generic case > > it may be false. It may depend on stack size and location. > > > > For instance, with large structs declared either at the top > > of a function or in separate branches within the function: > > I think you realize your example is way more complicated than > initializing basic type variable to 0 or NULL which has probably been > optimized forever. Maybe. Markus does a lot of what most (or perhaps some) consider thoughtless. A generic point might be maximizing the local scope of declarations. Maximizing the local scope can make reading easier as the type of an automatic used for a temporary purpose can be found closer to the code that uses it. This is especially true for long line-count functions. IMO: it's a reasonable goal though in this instance perhaps unnecessary. cheers, Joe
On Fri, Dec 11, 2020 at 10:08:54AM +0100, Markus Elfring wrote: > > This makes it hard to review any patches or follow discussion... > > You shared also special software development opinions about extra variable > initialisations occasionally, didn't you? I generally put everything at the top of the function... I don't have a well developed philosophy for when variables should be declared with a smaller scope. int ret; <-- this should always be function scope Probably the other people are right that making scopes shorter is more important when the function is very long. regards, dan carpenter
> Probably the other people are right that making scopes shorter is more > important when the function is very long. How much will sizes of function implementations influence the acceptance for possible source code refactorings? https://refactoring.com/catalog/reduceScopeOfVariable.html * Would you find another conversion of an extra null pointer initialisation to a later variable definition helpful then at other places? Example: [PATCH] dm snapshot: Reduce scope for the variable “pe” in snapshot_map() https://elixir.bootlin.com/linux/v5.10/source/drivers/md/dm-snap.c#L1941 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm-snap.c?id=2c85ebc57b3e1817b6ce1a6b703928e113a90442#n1941 * How do you think about to omit the initialisation for the variable “ret” in the implementation of the function “w1_uevent” (because it will be reassigned by a function call a bit later)? https://elixir.bootlin.com/linux/v5.10/source/drivers/w1/w1.c#L578 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/w1/w1.c?id=2c85ebc57b3e1817b6ce1a6b703928e113a90442#n578 Would any adjustment with the tag “Addresses-Coverity: ("Unused value")” become relevant here? Regards, Markus
diff --git a/drivers/mmc/host/atmel-mci.c b/drivers/mmc/host/atmel-mci.c index 444bd3a0a922..6a0d999ee82e 100644 --- a/drivers/mmc/host/atmel-mci.c +++ b/drivers/mmc/host/atmel-mci.c @@ -1558,7 +1558,6 @@ static void atmci_request_end(struct atmel_mci *host, struct mmc_request *mrq) __releases(&host->lock) __acquires(&host->lock) { - struct atmel_mci_slot *slot = NULL; struct mmc_host *prev_mmc = host->cur_slot->mmc; WARN_ON(host->cmd || host->data); @@ -1579,8 +1578,9 @@ static void atmci_request_end(struct atmel_mci *host, struct mmc_request *mrq) host->cur_slot->mrq = NULL; host->mrq = NULL; if (!list_empty(&host->queue)) { - slot = list_entry(host->queue.next, - struct atmel_mci_slot, queue_node); + struct atmel_mci_slot *slot = list_entry(host->queue.next, + struct atmel_mci_slot, + queue_node); list_del(&slot->queue_node); dev_vdbg(&host->pdev->dev, "list not empty: %s is next\n", mmc_hostname(slot->mmc));