Consider the function:

  extern (C++) int insbad(int a, int b) {
    asm {
	naked            ;
	mov EAX,a-4+[ESP] ;
	mov EBX,b-4+[ESP] ;
    }
  }

In 2.079 and earlier, the inline assembler assumed that an EBP was set up and provided offsets to the parameters accordingly. Hence, in the above code, a -4 was inserted to correct for not having EBP pushed on the stack.

Somewhere between 2.079 and 2.090 this was changed to not consider EBP, and the offsets look like this for the above function:

  ?insbad@@YAHHH@Z:
         mov	EAX,[ESP]    // should be 4[ESP]
	 mov	EBX,4[ESP]   // should be 8[ESP]

The problem looks to be the computation of Para.size in cgcod.d from this PR:

 https://github.com/dlang/dmd/pull/9620/files

i.e. my fault.