为何TCC会生成冗余NOP及空跳转JMP指令？

First, let’s confirm your hunch about the NOP instructions: you’re absolutely right—they’re almost certainly for memory alignment. On 32-bit x86 systems, aligning key code sections (like function start points or critical instruction sequences) to 16-byte boundaries can improve CPU cache hit rates. TCC, even in its lightweight design, includes basic alignment logic to avoid performance hits from misaligned memory accesses.

Now, onto that confusing "jump to the next instruction" JMP—this is a quirk of TCC’s older code generator, especially in version 0.9.26. Here’s why it might happen:

• Simplified Code Generation Templates: TCC prioritizes fast compilation and small binary size over aggressive optimizations. Its code generator uses reusable templates for common code patterns, like function exit paths. In some cases, even when the jump is redundant (pointing directly to the LEAVE instruction that would execute anyway), the template doesn’t include logic to eliminate it. It’s easier for the compiler to emit the jump unconditionally than to add checks to remove it in edge cases.
• Legacy Compatibility or Unfinished Optimizations: Older versions of TCC (like 0.9.26, which is quite dated) might not have implemented redundant branch elimination for certain code paths. Modern compilers (GCC, Clang, even newer TCC versions) would detect this useless jump and strip it out, but TCC’s minimalistic design skips some of these optimizations to keep compilation fast.
• Branch Prediction Resilience: Even though the JMP seems pointless, modern x86 CPUs’ branch predictors will recognize it as a no-op and execute it with zero overhead. So while it looks odd in the assembly, it doesn’t actually hurt runtime performance.

In short: the NOPs are intentional for alignment, and the redundant JMP is a side effect of TCC’s lightweight, template-based code generation that skips some optimization passes to prioritize speed and simplicity.

内容的提问来源于stack exchange，提问作者alexstx