> On May 12, 2016, at 4:01 PM, Kinney, Michael D <[email protected]> > wrote: > > Leif, > > Yes. Typecasts are used by the caller if the type does not match. Here is > an > example from the PciCfg2.c that you referenced. > > IN OUT VOID *Buffer > > // > // Aligned Pci address access > // > WriteUnaligned16 (((UINT16 *) Buffer), PciRead16 (PciLibAddress)); > > > Since Buffer is type VOID *, a typecast to UINT16 * is made. That is valid C > code.
Mike, Leif may be talking about: The C Standard, 6.3.2.3, paragraph 7 [ISO/IEC 9899:2011], states: A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined. I always like to reference Chris Lattner's LLVM blog post on undefined behavior, but this is not an example of the specific case. But the restrictions on type conversions exist to enable TBAA. http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html Violating Type Rules: It is undefined behavior to cast an int* to a float* and dereference it (accessing the "int" as if it were a "float"). C requires that these sorts of type conversions happen through memcpy: using pointer casts is not correct and undefined behavior results. The rules for this are quite nuanced and I don't want to go into the details here (there is an exception for char*, vectors have special properties, unions change things, etc). This behavior enables an analysis known as "Type-Based Alias Analysis" (TBAA) which is used by a broad range of memory access optimizations in the compiler, and can significantly improve performance of the generated code. For example, this rule allows clang to optimize this function: float *P; void zero_array() { int i; for (i = 0; i < 10000; ++i) P[i] = 0.0f; } into "memset(P, 0, 40000)". This optimization also allows many loads to be hoisted out of loops, common subexpressions to be eliminated, etc. This class of undefined behavior can be disabled by passing the -fno-strict-aliasing flag, which disallows this analysis. When this flag is passed, Clang is required to compile this loop into 10000 4-byte stores (which is several times slower), because it has to assume that it is possible for any of the stores to change the value of P, as in something like this: int main() { P = (float*)&P; // cast causes TBAA violation in zero_array. zero_array(); } This sort of type abuse is pretty uncommon, which is why the standard committee decided that the significant performance wins were worth the unexpected result for "reasonable" type casts. It is worth pointing out that Java gets the benefits of type-based optimizations without these drawbacks because it doesn't have unsafe pointer casting in the language at all. Thanks, Andrew Fish > One use case where making these APIs type specific is when a data structure > is used that is either byte packed or the start of the structure is not > aligned (e.g. > network packet processing). In these cases, the fields may have explicit > types such > as UINT16, UINT32, UINT64, and the compiler does type checking when a pointer > to the > field is passed into the unaligned access functions. > > By changing the APIs to be VOID* instead of UINT16*, UINT32*, or UINT64*, > some type > checking is lost. The APIs as they are defines now have worked for IPF. > What is > different about ARM and AArch64? Are you wanting to remove the requirement > for the > explicit typecast by the caller? > > > > You are correct. PatchCheck.py does not check EDK II C style. The ECC tool > can be > used for that. We have discussed adding ECC to PatchCheck.py, but have not > done that > yet. > > > > Thanks, > > Mike > > > > >> -----Original Message----- >> From: Leif Lindholm [mailto:[email protected]] >> Sent: Thursday, May 12, 2016 3:49 PM >> To: Kinney, Michael D <[email protected]> >> Cc: [email protected]; Gao, Liming <[email protected]>; Ard >> Biesheuvel >> <[email protected]> >> Subject: Re: [RFC] MdePkg: BaseLib: don't use aligned pointers for unaligned >> accessors >> >> On Thu, May 12, 2016 at 10:29:34PM +0000, Kinney, Michael D wrote: >>> Leif, >>> >>> Why does the current definition strictly require alignment? >>> >>> I know we have used this for a long time with many compilers passing in >>> unaligned pointers to UINT16, UINT32, UINT64 values without any compiler >>> warnings/errors. >> >> Sure, but that does not make it valid C. >> >> Specifying an input argument to be a UINT32 * at least in ARM, AArch64 >> and IPF land means that the programmer provides an explicit guarantee >> that the pointer references a naturally aligned location. >> >> Also, the reason that edk2 compiles without warnings is that code in >> the tree makes (also invalid) casts at the call sites to these >> functions in order to get rid of said warnings. See for example >> MdeModulePkg/Universal/PcatSingleSegmentPciCfg2Pei/PciCfg2.c >> >> (A naive grep throws up 129 instances of explicit casts on lines >> calling a ReadUnaligned function and 211 instances of ones calling a >> WriteUnaligned function.) >> >>> Also, the patch contains locals that are initialized in the declaration >>> of the local. We prefer locals to be initialized in the body of the >>> function. >> >> Understood - PatchCheck.py didn't warn me about that :) >> >> Regards, >> >> Leif >> >>> Thanks, >>> >>> Mike >>> >>>> -----Original Message----- >>>> From: Leif Lindholm [mailto:[email protected]] >>>> Sent: Thursday, May 12, 2016 3:22 PM >>>> To: [email protected] >>>> Cc: Kinney, Michael D <[email protected]>; Gao, Liming >>>> <[email protected]>; Ard Biesheuvel <[email protected]> >>>> Subject: [RFC] MdePkg: BaseLib: don't use aligned pointers for unaligned >> accessors >>>> >>>> The ReadUnaligned##/WriteUnaligned## functions provide a portable way of >>>> accessing potentially unaligned locations, but the prototypes in >>>> BaseLib.h all specify strictly aligned pointers. >>>> >>>> Change the prototypes as well as the implementations across Ia32/X64, >>>> ARM/AArch64 and IPF to use VOID * pointers instead. >>>> >>>> Contributed-under: TianoCore Contribution Agreement 1.0 >>>> Signed-off-by: Leif Lindholm <[email protected]> >>>> --- >>>> >>>> Built and tested for ArmVirtPkg for AArch64/ARM DEBUG/RELEASE. >>>> Build tested only for OvmfPkg for Ia32/X64 DEBUG/RELEASE. >>>> Not tested at all for IPF. >>>> >>>> I am not entirely convinced by the BitFieldWrite32 logic in >>>> WriteUnaligned24, but this patch is not the place to fix this. >>>> >>>> I am also not convinced there is much point in keeping separate >>>> implementations for IPF and ARM* - the optimization issues mentioned >>>> in the git log could even have been affected by the incorrect >>>> pointer types. >>>> >>>> This is the first patch triggered by my CLANG -Weverything exercises >>>> earlier this year. More to come. >>>> >>>> MdePkg/Include/Library/BaseLib.h | 16 ++++++------ >>>> MdePkg/Library/BaseLib/Arm/Unaligned.c | 16 ++++++------ >>>> MdePkg/Library/BaseLib/Ipf/Unaligned.c | 16 ++++++------ >>>> MdePkg/Library/BaseLib/Unaligned.c | 48 >>>> ++++++++++++++++++++++------------ >>>> 4 files changed, 56 insertions(+), 40 deletions(-) >>>> >>>> diff --git a/MdePkg/Include/Library/BaseLib.h >>>> b/MdePkg/Include/Library/BaseLib.h >>>> index c41fa78..133656a 100644 >>>> --- a/MdePkg/Include/Library/BaseLib.h >>>> +++ b/MdePkg/Include/Library/BaseLib.h >>>> @@ -2549,7 +2549,7 @@ DivS64x64Remainder ( >>>> UINT16 >>>> EFIAPI >>>> ReadUnaligned16 ( >>>> - IN CONST UINT16 *Buffer >>>> + IN CONST VOID *Buffer >>>> ); >>>> >>>> >>>> @@ -2571,7 +2571,7 @@ ReadUnaligned16 ( >>>> UINT16 >>>> EFIAPI >>>> WriteUnaligned16 ( >>>> - OUT UINT16 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT16 Value >>>> ); >>>> >>>> @@ -2592,7 +2592,7 @@ WriteUnaligned16 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned24 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ); >>>> >>>> >>>> @@ -2614,7 +2614,7 @@ ReadUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned24 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ); >>>> >>>> @@ -2635,7 +2635,7 @@ WriteUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned32 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ); >>>> >>>> >>>> @@ -2657,7 +2657,7 @@ ReadUnaligned32 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned32 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ); >>>> >>>> @@ -2678,7 +2678,7 @@ WriteUnaligned32 ( >>>> UINT64 >>>> EFIAPI >>>> ReadUnaligned64 ( >>>> - IN CONST UINT64 *Buffer >>>> + IN CONST VOID *Buffer >>>> ); >>>> >>>> >>>> @@ -2700,7 +2700,7 @@ ReadUnaligned64 ( >>>> UINT64 >>>> EFIAPI >>>> WriteUnaligned64 ( >>>> - OUT UINT64 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT64 Value >>>> ); >>>> >>>> diff --git a/MdePkg/Library/BaseLib/Arm/Unaligned.c >>>> b/MdePkg/Library/BaseLib/Arm/Unaligned.c >>>> index 34f1732..ef9efbc 100644 >>>> --- a/MdePkg/Library/BaseLib/Arm/Unaligned.c >>>> +++ b/MdePkg/Library/BaseLib/Arm/Unaligned.c >>>> @@ -33,7 +33,7 @@ >>>> UINT16 >>>> EFIAPI >>>> ReadUnaligned16 ( >>>> - IN CONST UINT16 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> volatile UINT8 LowerByte; >>>> @@ -65,7 +65,7 @@ ReadUnaligned16 ( >>>> UINT16 >>>> EFIAPI >>>> WriteUnaligned16 ( >>>> - OUT UINT16 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT16 Value >>>> ) >>>> { >>>> @@ -93,7 +93,7 @@ WriteUnaligned16 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned24 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> ASSERT (Buffer != NULL); >>>> @@ -122,7 +122,7 @@ ReadUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned24 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> @@ -149,7 +149,7 @@ WriteUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned32 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> UINT16 LowerBytes; >>>> @@ -181,7 +181,7 @@ ReadUnaligned32 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned32 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> @@ -208,7 +208,7 @@ WriteUnaligned32 ( >>>> UINT64 >>>> EFIAPI >>>> ReadUnaligned64 ( >>>> - IN CONST UINT64 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> UINT32 LowerBytes; >>>> @@ -240,7 +240,7 @@ ReadUnaligned64 ( >>>> UINT64 >>>> EFIAPI >>>> WriteUnaligned64 ( >>>> - OUT UINT64 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT64 Value >>>> ) >>>> { >>>> diff --git a/MdePkg/Library/BaseLib/Ipf/Unaligned.c >>>> b/MdePkg/Library/BaseLib/Ipf/Unaligned.c >>>> index 7d0d8dd..5ba8029 100644 >>>> --- a/MdePkg/Library/BaseLib/Ipf/Unaligned.c >>>> +++ b/MdePkg/Library/BaseLib/Ipf/Unaligned.c >>>> @@ -30,7 +30,7 @@ >>>> UINT16 >>>> EFIAPI >>>> ReadUnaligned16 ( >>>> - IN CONST UINT16 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> ASSERT (Buffer != NULL); >>>> @@ -56,7 +56,7 @@ ReadUnaligned16 ( >>>> UINT16 >>>> EFIAPI >>>> WriteUnaligned16 ( >>>> - OUT UINT16 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT16 Value >>>> ) >>>> { >>>> @@ -84,7 +84,7 @@ WriteUnaligned16 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned24 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> ASSERT (Buffer != NULL); >>>> @@ -113,7 +113,7 @@ ReadUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned24 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> @@ -140,7 +140,7 @@ WriteUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned32 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> UINT16 LowerBytes; >>>> @@ -172,7 +172,7 @@ ReadUnaligned32 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned32 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> @@ -199,7 +199,7 @@ WriteUnaligned32 ( >>>> UINT64 >>>> EFIAPI >>>> ReadUnaligned64 ( >>>> - IN CONST UINT64 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> UINT32 LowerBytes; >>>> @@ -231,7 +231,7 @@ ReadUnaligned64 ( >>>> UINT64 >>>> EFIAPI >>>> WriteUnaligned64 ( >>>> - OUT UINT64 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT64 Value >>>> ) >>>> { >>>> diff --git a/MdePkg/Library/BaseLib/Unaligned.c >> b/MdePkg/Library/BaseLib/Unaligned.c >>>> index 68dafa6..ed58861 100644 >>>> --- a/MdePkg/Library/BaseLib/Unaligned.c >>>> +++ b/MdePkg/Library/BaseLib/Unaligned.c >>>> @@ -32,12 +32,14 @@ >>>> UINT16 >>>> EFIAPI >>>> ReadUnaligned16 ( >>>> - IN CONST UINT16 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> + CONST UINT16 *Input = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer; >>>> + return *Input; >>>> } >>>> >>>> /** >>>> @@ -58,13 +60,15 @@ ReadUnaligned16 ( >>>> UINT16 >>>> EFIAPI >>>> WriteUnaligned16 ( >>>> - OUT UINT16 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT16 Value >>>> ) >>>> { >>>> + UINT16 *Output = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer = Value; >>>> + return *Output = Value; >>>> } >>>> >>>> /** >>>> @@ -83,12 +87,14 @@ WriteUnaligned16 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned24 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> + CONST UINT32 *Input = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer & 0xffffff; >>>> + return *Input & 0xffffff; >>>> } >>>> >>>> /** >>>> @@ -109,13 +115,15 @@ ReadUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned24 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> + UINT32 *Output = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - *Buffer = BitFieldWrite32 (*Buffer, 0, 23, Value); >>>> + *Output = BitFieldWrite32 (*Output, 0, 23, Value); >>>> return Value; >>>> } >>>> >>>> @@ -135,12 +143,14 @@ WriteUnaligned24 ( >>>> UINT32 >>>> EFIAPI >>>> ReadUnaligned32 ( >>>> - IN CONST UINT32 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> + CONST UINT32 *Input = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer; >>>> + return *Input; >>>> } >>>> >>>> /** >>>> @@ -161,13 +171,15 @@ ReadUnaligned32 ( >>>> UINT32 >>>> EFIAPI >>>> WriteUnaligned32 ( >>>> - OUT UINT32 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT32 Value >>>> ) >>>> { >>>> + UINT32 *Output = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer = Value; >>>> + return *Output = Value; >>>> } >>>> >>>> /** >>>> @@ -186,12 +198,14 @@ WriteUnaligned32 ( >>>> UINT64 >>>> EFIAPI >>>> ReadUnaligned64 ( >>>> - IN CONST UINT64 *Buffer >>>> + IN CONST VOID *Buffer >>>> ) >>>> { >>>> + CONST UINT64 *Input = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer; >>>> + return *Input; >>>> } >>>> >>>> /** >>>> @@ -212,11 +226,13 @@ ReadUnaligned64 ( >>>> UINT64 >>>> EFIAPI >>>> WriteUnaligned64 ( >>>> - OUT UINT64 *Buffer, >>>> + OUT VOID *Buffer, >>>> IN UINT64 Value >>>> ) >>>> { >>>> + UINT64 *Output = Buffer; >>>> + >>>> ASSERT (Buffer != NULL); >>>> >>>> - return *Buffer = Value; >>>> + return *Output = Value; >>>> } >>>> -- >>>> 2.1.4 >>> > _______________________________________________ > edk2-devel mailing list > [email protected] > https://lists.01.org/mailman/listinfo/edk2-devel _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
