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?