Ghidra's pseudocode of a Watcom function leaves out arguments and stores that the disassembly shows

Symptoms

A function's pseudocode shows fewer arguments than its callers pass, or reads registers that were never set. A value the port computes from the pseudocode is wrong, and nothing in the pseudocode explains why. A string that the program obviously uses has no user in decomp.c.

In Hardwar the pseudocode of crater_lights_place (0x44b9c8) showed each light's colour and radius being passed on, so the port drew the lights at their stored height of 0, below the crater floors. The disassembly showed that the function copies the stored position to a vector on the stack, writes the radius shifted right by 3 over its height, and passes that vector in a register to the next call (observed).

Cause

Watcom C passes the first four arguments in EAX, EDX, EBX and ECX (__watcall). Ghidra's default convention for a 32-bit Windows program passes them on the stack. With the wrong convention the decompiler does not see the register arguments, so it drops them and treats the stores that fill them as dead. A pointer to a string loaded into EAX before a call is lost the same way (observed in Hardwar and in POD).

What to do

For any Watcom function the port depends on, read the disassembly and check every register set before each call. The pseudocode is a guide to the shape of the function only.

./gameframe run ghidra-run GAME.EXE disasm analysis/notes/lights.asm 0x44b9c8        # the whole function
./gameframe run ghidra-run GAME.EXE disasm analysis/notes/span.asm 0x44b9c8-0x44ba40 # a range, in order
./gameframe run refs GAME.EXE 0x535f43 --functions analysis/export/GAME.EXE/functions.tsv   # users of a string
./gameframe run ghidra-run GAME.EXE callargs analysis/notes/srand.tsv regs=watcom srand  # every call's EAX, EDX, EBX, ECX

callargs regs=watcom reads, for every call to a function, the constant each register gets before the call in the same block. It finds the seeds, ids and sizes a Watcom program passes in registers, which the pseudocode leaves out.

Set the calling convention of the functions you trace to __watcall in Ghidra, or give them custom storage, and decompile again (docs/working/ghidra.md). Record the real arguments in the topic document. Keep the .asm files in analysis/: they reproduce the program's code.

Tried

• Trusting the pseudocode's argument list: the crater lights hung at height 0 until the disassembly was read.