C5500 Compiler Build Options One Use Case

Transcription

1 C5500 Compiler Build Options One Use Case May 2008 Page 1 Page 1 TI Internal Use Only

2 Overview These slides are the collective wisdom on C5500 Compiler Build options from a highly experienced development team within TI Many points, however, apply equally well to other TI compilers Page 2

3 Goals Understand impact of critical C compiler options with regards to: Code size CPU cycles Compare CGT 3.x vs 4.x Benchmarks performed with real world DSP algorithms, not sample code! Page 3

4 Test environment DSP Module tested (dec/enc)oding, (de)interleaving Optimized code using Viterbi HWA + hand assembly code C version: only C code, including a C model for Viterbi CGT and CCS v3.3 C55 Rev 3.0 Cycle accurate simulator Compiler options configuration set: Default None --symdebug:dwarf -O2 -mn ms No -ms No -mn No debug With O3 --symdebug:dwarf -O2 -mn --symdebug:dwarf -O2 ms -O2 ms --symdebug:dwarf O3 -mn ms Page 4

5 Notes Lots of data generated. The following slides give key take-away --symdebug:dwarf is equivalent to -g since CGT The fully qualified name is used for clarity. Page 5

6 Key Result 1: -mn option is a must-have ~7% CPU cycles improvements when combined with -O2 -g Improves debug-ability of code Minor impact on code size ~1% Available since CGT 2.70 Using the -mn (normal optimization with debug) option in conjunction with the debug option (-g) and the optimization option (-o[=0-3]) allows the compiler to perform certain optimizations that are normally disabled when compiling with the debug option. The class of optimizations affected is those which result in "code motion", that is, a re-organization of the generated code for a series of source statements such that the order of the generated code does not occur in the same sequence as the source code. Such re-organization can result in various performance improvements such as allowing additional parallelism in the object code. The price of the resulting improved performance is that the debugger may sometimes behave in curious ways. For example, when doing single assembly steps in the debugger, the highlighting bar in the source window may jump wildly from one source line to another rather than proceeding through the expected sequence of source lines. Page 6

7 No mn for C6000 Users C6000 Compiler Does Not Support mn Use --optimize_with_debug instead Undocumented option Available since v5.1.0 Page 7

8 Key result 2: -ms usage for CGT 4.x -ms definition changes in CGT 4.x. It takes a new numerical argument to indicate how strong the user's preference is for optimizing for space rather than speed. -ms with no argument is equivalent to -ms3 The -ms value may also determine the degree to which certain optimizations are applied. For example, placing instructions in parallel improves the performance of the code, but results in an additional byte of code. At -ms3 (total commitment to space savings) no instructions will be placed in parallel resulting in space savings but with a likely price of slower execution. At -ms2 instructions will be placed in parallel only if they occur inside a loop. This allows space to be saved in straight line code without slowing down looping code (which is likely to determine the overall program performance). The following table summarizes the -ms[=0-3] option: Option Strategy Examples no -ms total commitment All loop optimizations regardless of impact to speed on size -ms0 speed more critical Loop optimizations with larger size cost -ms1 speed critical Loop optimizations with modest size cost -ms2 size more critical Parallel instruction pairs only in loops -ms3 total commitment No parallel instruction pairs to size Page 8

9 Key result 2: -ms usage for CGT 4.x (2) CPU cycles in bytes Effects of msn vary between modules Typically adjust for control code vs. algorithm code Page 9

10 Miscellaneous results: -O2 and ms to be removed in assembler options -O3 is not as good as expected. This option would provide better results if used with -pm to allow program-level optimizations. This is not supported by our current build system. Page 10

11 Debug-ability When using compiler optimizations it can become difficult to debug source code with CCS PC arrow jumping randomly from one statement to the other PC arrow skipping C statement Using -mn improves debug-ability: No debug for C, sym:dwarf for ASM only, no -mn No debug for C, No debug for ASM, no - mn Debug for C, No debug for ASM, -mn, -s sym:dwarf debug for C, sym:dwarf for ASM, no -mn sym:dwarf debug for C, sym:dwarf for ASM, -mn, no -ms Breakpoint in ASM Breakpoint in C Stepping in ASM Stepping in C Profiling Memory window for globals Mixed Mode Watch Window Yes yes at the Yes No, no yes yes Yes Issues top of the step into C function only. Yes yes at the No No, no yes yes Yes Issues top of the step into C function only. yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes Page 11

12 Conclusion Recommend options for best overall performance for C55 : --symdebug:dwarf -O2 -mn ms Page 12