Implement dead code elimination #145
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jserv
reviewed
Aug 6, 2024
jserv
reviewed
Aug 6, 2024
jserv
reviewed
Aug 6, 2024
jserv
reviewed
Aug 8, 2024
jserv
reviewed
Aug 8, 2024
|
I defer to @vacantron for confirmation. |
jserv
requested changes
Aug 9, 2024
|
Once the DCE is known to work properly, we can close #88 to declare the capability of performing basic optimizations. |
This error occurred when a return statement was in for-loop body. After parsing loop body, "body_" was assigned NULL as the return value, causing the compiler to failed in correctly connect "inc_" and "cond_start" basic blocks. This error can be analyzed by examining the CFG.dot file.
Introduces the creaetion of reverse dominance frontier (RDF) to support dead code elimination (DCE). Method for implementing RDF is similar to that of dominance frontier (DF). The key difference is that RDF is computed on the reverse CFG. In other words, operations were performed on "prev[]" which in the basic block structure, now switched to operate on "next", "then_" and "else_". In the "dce_insn" function, mark useful instructions during the initial analysis of the current basic block. Continue identifying useful instructions by tracing back from the last assigned instruction of the both operands of the current "insn". In the "dce_sweep" function remove the useless instruction from the current "insn_list". If a branch instruction is encountered, remove it and reconnect the current basic block to its reverse immediate dominator. Before implementing DCE, compiling "src/main.c" resulted in an executable with 51,357 instructions for ARMv7-A. After DCE, the executable was 51,330 instructions, reducing the total by 27 instructions.
vacantron
approved these changes
Aug 14, 2024
|
Thank @nosba0957 for contributing! |
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The creation of RDF is based on reverse CFG. The method of building RDF is similar to building DF, but instead of iterating every
prevnodes, it visitnext,then_andelse_nodes. The code appears to extend the for-loop to handle three cases (compared to the method of building DF).Due to the addition of RDF, several elements were added to the
struct basic_block. However,MAX_FILEDmacro is not large enough, so it has been adjusted to 64.DCE is implemented using mark-sweep algorithm.
dce_insnmarks the useful instruction anddce_sweepremoves the operations that are not marked "useful". Thedce_insnmust execute aftercseandconst_foldingare done becausedce_insnmay mark some instructions that are not handled bycseandconst_folding. These instruction might be mistakenly mark "useful".Related issue: #88