Ethernet Swindle and gcc vs clang

 There will be soon a new flavor of the swindle : The ethernet swindle


It's starting to work nicely, utltimately the goal is to use the CH32V208 that is available for ~ 4$ on aliexpress. It's basically a CH32V307 without HS usb and without fpu,  same flash, same ram (more on that later).


 LWIP is pretty big and i'm running short on both flash & ram.

While looking into it, i discovered a nice option that gcc has and clang does not

"-msave-restore"

Since the Riscv does not have stmia/ldmia style instruction, it must push and pop all registers one at a time when entering/exiting a function. That is consuming a lot of code space for short functions.

The -msave-restore creates function to save/restore , all possible variants and call them instead of manually pushing/popping registers.

Let's give it a try, baseline is clang + hw FPU:

Clang + HW FPU    255 508   (+0kB)

Clang + no FPU       257 632  (+2KB)

Clang + FPU+LTO  232 690  (-22kB)

GCC + no FPU        258 934 (+3kB)

GCC+no FPU+saverestore 244 000 (-14 kB)


LTO is the big winner but it makes the code difficult to debug and is prone to quirky behaviour.

The "-msave-restore" gives a nice 14kB decrease. Too bad it's not available for clang.     


Comments

  1. AdamFebruary 11, 2026 at 5:08 AM

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