| Message ID | 20190327184531.30986-8-julien.grall@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | xen/arm: Add support to build with clang | expand |
On Wed, 27 Mar 2019, Julien Grall wrote: > Clang is pickier than GCC for the register size in asm statement. It > expects the register size to match the value size. > > The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 > (resp. Arm64) whereas the value is a boolean (Clang consider to be > 32-bit). > > It would be possible to impose 32-bit register for both architecture > but this require the code to use __OP32. However, it does not really > improve the assembly generated. Instead, replace switch the variable > to use register_t. > > Signed-off-by: Julien Grall <julien.grall@arm.com> > --- > xen/include/asm-arm/cpuerrata.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h > index 55ddfda272..88ef3ca934 100644 > --- a/xen/include/asm-arm/cpuerrata.h > +++ b/xen/include/asm-arm/cpuerrata.h > @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) \ > return false; \ > else \ > { \ > - bool ret; \ > + register_t ret; \ > \ > asm volatile (ALTERNATIVE("mov %0, #0", \ > "mov %0, #1", \ This is OK. Could you please also change the return statement below? Maybe something like: return unlikely(!!ret); Thank you!
Hi, On 4/17/19 9:28 PM, Stefano Stabellini wrote: > On Wed, 27 Mar 2019, Julien Grall wrote: >> Clang is pickier than GCC for the register size in asm statement. It >> expects the register size to match the value size. >> >> The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 >> (resp. Arm64) whereas the value is a boolean (Clang consider to be >> 32-bit). >> >> It would be possible to impose 32-bit register for both architecture >> but this require the code to use __OP32. However, it does not really >> improve the assembly generated. Instead, replace switch the variable >> to use register_t. >> >> Signed-off-by: Julien Grall <julien.grall@arm.com> >> --- >> xen/include/asm-arm/cpuerrata.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h >> index 55ddfda272..88ef3ca934 100644 >> --- a/xen/include/asm-arm/cpuerrata.h >> +++ b/xen/include/asm-arm/cpuerrata.h >> @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) \ >> return false; \ >> else \ >> { \ >> - bool ret; \ >> + register_t ret; \ >> \ >> asm volatile (ALTERNATIVE("mov %0, #0", \ >> "mov %0, #1", \ > > This is OK. Could you please also change the return statement below? > Maybe something like: > > return unlikely(!!ret); Why? The compiler will implicitly convert the int to bool. 0 will turn to false, all the other will be true. We actually been actively removing !! when the type is bool (see the example in get_paged_frame in common/grant_table.c). Cheers,
On Wed, 17 Apr 2019, Julien Grall wrote: > Hi, > > On 4/17/19 9:28 PM, Stefano Stabellini wrote: > > On Wed, 27 Mar 2019, Julien Grall wrote: > > > Clang is pickier than GCC for the register size in asm statement. It > > > expects the register size to match the value size. > > > > > > The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 > > > (resp. Arm64) whereas the value is a boolean (Clang consider to be > > > 32-bit). > > > > > > It would be possible to impose 32-bit register for both architecture > > > but this require the code to use __OP32. However, it does not really > > > improve the assembly generated. Instead, replace switch the variable > > > to use register_t. > > > > > > Signed-off-by: Julien Grall <julien.grall@arm.com> > > > --- > > > xen/include/asm-arm/cpuerrata.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/xen/include/asm-arm/cpuerrata.h > > > b/xen/include/asm-arm/cpuerrata.h > > > index 55ddfda272..88ef3ca934 100644 > > > --- a/xen/include/asm-arm/cpuerrata.h > > > +++ b/xen/include/asm-arm/cpuerrata.h > > > @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) > > > \ > > > return false; \ > > > else \ > > > { \ > > > - bool ret; \ > > > + register_t ret; \ > > > \ > > > asm volatile (ALTERNATIVE("mov %0, #0", \ > > > "mov %0, #1", \ > > > > This is OK. Could you please also change the return statement below? > > Maybe something like: > > > > return unlikely(!!ret); > Why? The compiler will implicitly convert the int to bool. 0 will turn to > false, all the other will be true. > > We actually been actively removing !! when the type is bool (see the example > in get_paged_frame in common/grant_table.c). Really? Too bad, I loved the explicit conversions to bool. This is a matter of code style, not correctness, so usually I wouldn't care much. But I went to read MISRA-C to figure out if there are any differences from that point of view. From Rule 10.3, it looks like it is not compliant, because they say that: bool_t bla = 0; is not MISRA-C compliant. While: int c = 1; bool_t bla = c == 0; is compliant. So, if I read this right: return !!ret //compliant return ret; //not compliant I am not 100% sure though.
Hi, On 18/04/2019 19:23, Stefano Stabellini wrote: > On Wed, 17 Apr 2019, Julien Grall wrote: >> Hi, >> >> On 4/17/19 9:28 PM, Stefano Stabellini wrote: >>> On Wed, 27 Mar 2019, Julien Grall wrote: >>>> Clang is pickier than GCC for the register size in asm statement. It >>>> expects the register size to match the value size. >>>> >>>> The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 >>>> (resp. Arm64) whereas the value is a boolean (Clang consider to be >>>> 32-bit). >>>> >>>> It would be possible to impose 32-bit register for both architecture >>>> but this require the code to use __OP32. However, it does not really >>>> improve the assembly generated. Instead, replace switch the variable >>>> to use register_t. >>>> >>>> Signed-off-by: Julien Grall <julien.grall@arm.com> >>>> --- >>>> xen/include/asm-arm/cpuerrata.h | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/xen/include/asm-arm/cpuerrata.h >>>> b/xen/include/asm-arm/cpuerrata.h >>>> index 55ddfda272..88ef3ca934 100644 >>>> --- a/xen/include/asm-arm/cpuerrata.h >>>> +++ b/xen/include/asm-arm/cpuerrata.h >>>> @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) >>>> \ >>>> return false; \ >>>> else \ >>>> { \ >>>> - bool ret; \ >>>> + register_t ret; \ >>>> \ >>>> asm volatile (ALTERNATIVE("mov %0, #0", \ >>>> "mov %0, #1", \ >>> >>> This is OK. Could you please also change the return statement below? >>> Maybe something like: >>> >>> return unlikely(!!ret); >> Why? The compiler will implicitly convert the int to bool. 0 will turn to >> false, all the other will be true. >> >> We actually been actively removing !! when the type is bool (see the example >> in get_paged_frame in common/grant_table.c). > > Really? Too bad, I loved the explicit conversions to bool. This is a > matter of code style, not correctness, so usually I wouldn't care much. > But I went to read MISRA-C to figure out if there are any differences > from that point of view. From Rule 10.3, it looks like it is not > compliant, because they say that: > > bool_t bla = 0; > > is not MISRA-C compliant. While: > > int c = 1; > bool_t bla = c == 0; > > is compliant. So, if I read this right: > > return !!ret //compliant > return ret; //not compliant > > I am not 100% sure though. And if you read that rule the following would also be non-compliant bool is_nonzero(int b) { return b; } I know this example is pretty exaggerated but then does it mean the following code is also non-compliant? bool is_nonzero(int b) { if (b) return true; else return false; } If it is considered compliant, then it does not make sense. Cheers,
On Thu, 18 Apr 2019, Julien Grall wrote: > Hi, > > On 18/04/2019 19:23, Stefano Stabellini wrote: > > On Wed, 17 Apr 2019, Julien Grall wrote: > > > Hi, > > > > > > On 4/17/19 9:28 PM, Stefano Stabellini wrote: > > > > On Wed, 27 Mar 2019, Julien Grall wrote: > > > > > Clang is pickier than GCC for the register size in asm statement. It > > > > > expects the register size to match the value size. > > > > > > > > > > The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 > > > > > (resp. Arm64) whereas the value is a boolean (Clang consider to be > > > > > 32-bit). > > > > > > > > > > It would be possible to impose 32-bit register for both architecture > > > > > but this require the code to use __OP32. However, it does not really > > > > > improve the assembly generated. Instead, replace switch the variable > > > > > to use register_t. > > > > > > > > > > Signed-off-by: Julien Grall <julien.grall@arm.com> > > > > > --- > > > > > xen/include/asm-arm/cpuerrata.h | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/xen/include/asm-arm/cpuerrata.h > > > > > b/xen/include/asm-arm/cpuerrata.h > > > > > index 55ddfda272..88ef3ca934 100644 > > > > > --- a/xen/include/asm-arm/cpuerrata.h > > > > > +++ b/xen/include/asm-arm/cpuerrata.h > > > > > @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) > > > > > \ > > > > > return false; \ > > > > > else \ > > > > > { \ > > > > > - bool ret; \ > > > > > + register_t ret; \ > > > > > \ > > > > > asm volatile (ALTERNATIVE("mov %0, #0", \ > > > > > "mov %0, #1", \ > > > > > > > > This is OK. Could you please also change the return statement below? > > > > Maybe something like: > > > > > > > > return unlikely(!!ret); > > > Why? The compiler will implicitly convert the int to bool. 0 will turn to > > > false, all the other will be true. > > > > > > We actually been actively removing !! when the type is bool (see the > > > example > > > in get_paged_frame in common/grant_table.c). > > > > Really? Too bad, I loved the explicit conversions to bool. This is a > > matter of code style, not correctness, so usually I wouldn't care much. > > But I went to read MISRA-C to figure out if there are any differences > > from that point of view. From Rule 10.3, it looks like it is not > > compliant, because they say that: > > > > bool_t bla = 0; > > > > is not MISRA-C compliant. While: > > > > int c = 1; > > bool_t bla = c == 0; > > > > is compliant. So, if I read this right: > > > > return !!ret //compliant > > return ret; //not compliant > > > > I am not 100% sure though. > > And if you read that rule the following would also be non-compliant > > bool is_nonzero(int b) > { > return b; > } Yes, I think you are right. > I know this example is pretty exaggerated but then does it mean the following > code is also non-compliant? > > bool is_nonzero(int b) > { > if (b) > return true; > else > return false; > } > > If it is considered compliant, then it does not make sense. Yes, I think this is not compliant too. Also, from what I have been told, this example is famous for being one of the most extreme examples of MISRA-C non-compliance. I think the compliant version would be: bool is_nonzero(int b) { if (b != 0) return true; else return false; } This is also compliant: bool is_nonzero(int b) { return (b != 0); }
On 18/04/2019 19:52, Stefano Stabellini wrote: > On Thu, 18 Apr 2019, Julien Grall wrote: >> On 18/04/2019 19:23, Stefano Stabellini wrote: >>> On Wed, 17 Apr 2019, Julien Grall wrote: >>>> On 4/17/19 9:28 PM, Stefano Stabellini wrote: >>>>> On Wed, 27 Mar 2019, Julien Grall wrote: >> If it is considered compliant, then it does not make sense. > > Yes, I think this is not compliant too. Also, from what I have been > told, this example is famous for being one of the most extreme examples > of MISRA-C non-compliance. I think the compliant version would be: > > bool is_nonzero(int b) > { > if (b != 0) > return true; > else > return false; > } > > This is also compliant: > > bool is_nonzero(int b) > { > return (b != 0); > } > I will use !!b here and in the next patch. But I still doubt you will be able to enforce it in Xen. if ( n ) is quite a common pattern. Cheers,
On Thu, 18 Apr 2019, Julien Grall wrote: > On 18/04/2019 19:52, Stefano Stabellini wrote: > > On Thu, 18 Apr 2019, Julien Grall wrote: > > > On 18/04/2019 19:23, Stefano Stabellini wrote: > > > > On Wed, 17 Apr 2019, Julien Grall wrote: > > > > > On 4/17/19 9:28 PM, Stefano Stabellini wrote: > > > > > > On Wed, 27 Mar 2019, Julien Grall wrote: > > > If it is considered compliant, then it does not make sense. > > > > Yes, I think this is not compliant too. Also, from what I have been > > told, this example is famous for being one of the most extreme examples > > of MISRA-C non-compliance. I think the compliant version would be: > > > > bool is_nonzero(int b) > > { > > if (b != 0) > > return true; > > else > > return false; > > } > > > > This is also compliant: > > > > bool is_nonzero(int b) > > { > > return (b != 0); > > } > > > > I will use !!b here and in the next patch. But I still doubt you will be able > to enforce it in Xen. if ( n ) is quite a common pattern. Thanks. Baby steps :-)
diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h index 55ddfda272..88ef3ca934 100644 --- a/xen/include/asm-arm/cpuerrata.h +++ b/xen/include/asm-arm/cpuerrata.h @@ -14,7 +14,7 @@ static inline bool check_workaround_##erratum(void) \ return false; \ else \ { \ - bool ret; \ + register_t ret; \ \ asm volatile (ALTERNATIVE("mov %0, #0", \ "mov %0, #1", \
Clang is pickier than GCC for the register size in asm statement. It expects the register size to match the value size. The asm statement expects a 32-bit (resp. 64-bit) value on Arm32 (resp. Arm64) whereas the value is a boolean (Clang consider to be 32-bit). It would be possible to impose 32-bit register for both architecture but this require the code to use __OP32. However, it does not really improve the assembly generated. Instead, replace switch the variable to use register_t. Signed-off-by: Julien Grall <julien.grall@arm.com> --- xen/include/asm-arm/cpuerrata.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)