On Die, 2013-08-20 at 13:16 -0400, Alex Deucher wrote:
> Newer versions of gcc seem to wander off into no-man's land
> when using variably sized arrays. Atombios tends to do things
> like:
>
> struct object {
> u8 version;
> u8 num_elements;
> u32 elements[1]; /* num_elements entries */
> };
>
> We then do things like the following in the driver code:
>
> for (i = 0; i < atom_object->num_elements; i++) {
> driver_object[i] = atom_object->elements[i];
> }
>
> With previous versions of gcc this used to work fine, but with
> 4.7 and 4.8, it seems to generate code that wanders off into the
> weeds.
>
> According to:
> http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> The traditional way of handling variably sized arrays (ISO C90)
> is to use an array length of 1. Gcc allows you to use an array
> length of 0 and newer versions of gcc only seem to do the
> right thing when 0 is used.
[...]
> diff --git a/drivers/gpu/drm/radeon/atombios.h b/drivers/gpu/drm/radeon/atombios.h
> index 16b120c..3f1f011 100644
> --- a/drivers/gpu/drm/radeon/atombios.h
> +++ b/drivers/gpu/drm/radeon/atombios.h
[...]
> @@ -4028,15 +4028,15 @@ typedef struct _ATOM_OBJECT_TABLE //Above 4 object table
[...]
> typedef struct _ATOM_SRC_DST_TABLE_FOR_ONE_OBJECT //usSrcDstTableOffset pointing to this structure
> {
> UCHAR ucNumberOfSrc;
> - USHORT usSrcObjectID[1];
> + USHORT usSrcObjectID[0];
> UCHAR ucNumberOfDst;
> - USHORT usDstObjectID[1];
> + USHORT usDstObjectID[0];
> }ATOM_SRC_DST_TABLE_FOR_ONE_OBJECT;
[...]
> @@ -6194,8 +6194,8 @@ typedef struct _ATOM_INIT_REG_INDEX_FORMAT{
> typedef struct _ATOM_INIT_REG_BLOCK{
> USHORT usRegIndexTblSize; //size of asRegIndexBuf
> USHORT usRegDataBlkSize; //size of ATOM_MEMORY_SETTING_DATA_BLOCK
> - ATOM_INIT_REG_INDEX_FORMAT asRegIndexBuf[1];
> - ATOM_MEMORY_SETTING_DATA_BLOCK asRegDataBuf[1];
> + ATOM_INIT_REG_INDEX_FORMAT asRegIndexBuf[0];
> + ATOM_MEMORY_SETTING_DATA_BLOCK asRegDataBuf[0];
> }ATOM_INIT_REG_BLOCK;
>
> #define END_OF_REG_INDEX_BLOCK 0x0ffff
> @@ -6938,8 +6938,8 @@ typedef struct _ATOM_DISP_OUT_INFO
> ATOM_COMMON_TABLE_HEADER sHeader;
> USHORT ptrTransmitterInfo;
> USHORT ptrEncoderInfo;
> - ASIC_TRANSMITTER_INFO asTransmitterInfo[1];
> - ASIC_ENCODER_INFO asEncoderInfo[1];
> + ASIC_TRANSMITTER_INFO asTransmitterInfo[0];
> + ASIC_ENCODER_INFO asEncoderInfo[0];
> }ATOM_DISP_OUT_INFO;
>
> typedef struct _ATOM_DISP_OUT_INFO_V2
> @@ -6948,8 +6948,8 @@ typedef struct _ATOM_DISP_OUT_INFO_V2
> USHORT ptrTransmitterInfo;
> USHORT ptrEncoderInfo;
> USHORT ptrMainCallParserFar; // direct address of main parser call in VBIOS binary.
> - ASIC_TRANSMITTER_INFO asTransmitterInfo[1];
> - ASIC_ENCODER_INFO asEncoderInfo[1];
> + ASIC_TRANSMITTER_INFO asTransmitterInfo[0];
> + ASIC_ENCODER_INFO asEncoderInfo[0];
> }ATOM_DISP_OUT_INFO_V2;
I'm not sure how these structs are supposed to work, before or after
this change... The compiler can't know how to access the fields after
the first variably-sized array?
--
Earthling Michel Dänzer | http://www.amd.com
Libre software enthusiast | Debian, X and DRI developer
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