Sorted in order of personal preference.
Note that picking your tool of choice by exact numbers is silly. A 1% gain here or there will not matter in a real-world scenario - stability and featuresets will matter much more. However, these are good in establishing a general ballpark of performance.
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Tested compilers: cc65, gcc-6502, KickC, lcc65, llvm-mos, sdcc, vbcc.
This is the most comprehensive one. Focuses on typical benchmark samples.
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Tested compilers: cc65, gcc-6502, KickC, llvm-mos (via pull request), vbcc.
Seems to mostly be based on real-world/tutorial cc65 code snippets. Some have optimized ASM to compare against.
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Tested compilers: cc65, gcc-6502, KickC, sdcc, vbcc.
Based on typical benchmark samples.
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Tested compilers: cc65, gcc-6502, KickC, llvm-mos, vbcc.
No methodology; in addition, NESFab is more akin to a high-level assembler than a C compiler.