[FAQ] TDA4VM: C7x compiler status on address beyond +/- 2GB

Reasoning

On C7000 devices, all symbol references are handled in a position-independent manner through a PC-relative offset. However, linker-defined symbols may have a value that does not resolve to a valid symbol address or resolves to an address that is beyond the +/- 2 GB reach of normal PC-relative addressing.

Let us consider a scenario to demonstrate this in J721e SDK 8.4

The appInit (@ 0xab669980) in the code section of C7x could reach upto 2GB (i.e. till 0x12B669980).

A variable g_ddr_local_mem (@ 0x12B700000) pointing to the C7X_1_LOCAL_HEAP_ADDR is being accessed by appInit. Since this variable is out of its reach, the below error is seen during compilation.

Linking /home/nikhil/SDK_8.4/ti-processor-sdk-rtos-j721e-evm-08_04_00_06/vision_apps/out/J7/C71/FREERTOS/release/vx_app_rtos_linux_c7x_1.out
"/home/nikhil/SDK_8.4/ti-processor-sdk-rtos-j721e-evm-08_04_00_06/ti-cgt-c7000_3.0.0.STS/include/string.h", line 275: error:
   relocation from function "appInit" to symbol "g_ddr_local_mem" overflowed;
   the 33-bit relocated address 0x80096600 is too large to encode in the 32-bit
   signed PC-Relative field (type = 'R_C7X_PCR_OFFSET_ADDKPC_HI27' (33), file =
   "/home/nikhil/SDK_8.4/ti-processor-sdk-rtos-j721e-evm-08_04_00_06/vision_app
   s/out/J7/C71/FREERTOS/release/app_rtos_common_c7x_1.lib<app_init.obj>",
   offset = 0x000000000084, section = ".text")
error: errors encountered during linking;
   "/home/nikhil/SDK_8.4/ti-processor-sdk-rtos-j721e-evm-08_04_00_06/vision_app
   s/out/J7/C71/FREERTOS/release/vx_app_rtos_linux_c7x_1.out" not built

Solution

When accessing such symbols, you must use the _symval() intrinsic to force the use of absolute addressing. Such addressing prevents the compiler from generating PC-relative offsets/relocations for symbols that don't actually correspond to addresses or to addresses that are beyond the +/- 2 GB reach of PC-relative addressing.

Considering our example scenario, we could use _symval(g_ddr_local_mem) inside appInit() function.

The following information is available in the SPRUIG8E_C7000_Compiler_Users_Guide.pdf available in the SDK in the section 12.6 Linker Symbols

${PSDKRA}/ti-cgt-c7000_3.0.0.STS/docs/SPRUIG8E_C7000_Compiler_Users_Guide.pdf

Regards,
Nikhil Dasan