83 lines
3.5 KiB
Plaintext
83 lines
3.5 KiB
Plaintext
|
.so init
|
||
|
|
.nr H1 2
|
||
|
|
.NH
|
||
|
|
PROBLEMS
|
||
|
|
.NH 2
|
||
|
|
Maintain SPARC speed
|
||
|
|
.PP
|
||
|
|
If we want to generate SPARC code, we should try to generate efficient code
|
||
|
|
as fast as possible. It would be quite embarrassing to find out that the
|
||
|
|
same program would run faster on a Motorola 68020 than on a SPARC processor,
|
||
|
|
when both operate at the same clock frequency.
|
||
|
|
Looking at some code generated by Sun's C-compiler and optimizing assembler,
|
||
|
|
we can spot a few remarkable characteristics of the generated SPARC code:
|
||
|
|
.IP -
|
||
|
|
There are almost no memory references
|
||
|
|
.IP -
|
||
|
|
Parameters to functions are passed through registers.
|
||
|
|
.IP -
|
||
|
|
Almost all delay slots\(dg
|
||
|
|
.FS
|
||
|
|
\(dg For details about delay slots see the SPARC Architecture Manual, chapter 4, pp. 42-48
|
||
|
|
.FE
|
||
|
|
are filled in by the assembler
|
||
|
|
.LP
|
||
|
|
If we want to generate efficient code, we should at least try to
|
||
|
|
reduce the number of memory references and use registers wherever we can.
|
||
|
|
Since EM is stack-oriented it references its stack for every operation so
|
||
|
|
this will not be an easy task; a suitable solution will however be given in
|
||
|
|
the next chapter.
|
||
|
|
.NH 2
|
||
|
|
Increase compilation speed
|
||
|
|
.PP
|
||
|
|
Because we will implement a code expander (fast backend) we should keep
|
||
|
|
a close eye on efficiency; if we cannot beat regular compilers on producing
|
||
|
|
efficient code we will try to beat them on fast code generation.
|
||
|
|
The usual trick to achieve fast compilation is to pack the frontend,
|
||
|
|
optimizer, code-generator and
|
||
|
|
assembler all into a single large binary to reduce the overhead of
|
||
|
|
reading and writing temporary files. Unfortunately, due to the
|
||
|
|
SPARC instruction set, its relocation information is slightly bizarre
|
||
|
|
and cannot be represented with the present primitives.
|
||
|
|
This means that it will not be possible to generate the required output
|
||
|
|
format directly from our backend.
|
||
|
|
.PP
|
||
|
|
There are three solutions here: generate assembler code, and let an
|
||
|
|
existing assembler generate the required object (\fI.o\fR) files,
|
||
|
|
create our own primitives than can handle the SPARC relocation format, or
|
||
|
|
do not use any of the addressing modes that require the bizarre relocation.
|
||
|
|
Because we have enough on our hands already we will
|
||
|
|
let the existing assembler deal with generating object files.
|
||
|
|
.NH 2
|
||
|
|
Convert stack to register operations
|
||
|
|
.PP
|
||
|
|
As we wrote in the previous chapter, for RISC machines a code expander can
|
||
|
|
produce almost as efficient code as a code generator. The fact that this is
|
||
|
|
true for stack-oriented RISC processors is rather obvious. The problem we
|
||
|
|
face, however, is that the SPARC processor is register, instead of
|
||
|
|
stack oriented. In the next chapter we will give a suitable solution to
|
||
|
|
convert most stack accesses to register accesses.
|
||
|
|
.NH 2
|
||
|
|
Miscellaneous
|
||
|
|
.PP
|
||
|
|
Besides performance and \fI.o\fR-compatibility there are some other
|
||
|
|
peculiarities of the SPARC processor and Sun's C-compiler (henceforth
|
||
|
|
simply called \fIcc\fR).
|
||
|
|
.PP
|
||
|
|
For some reason, the SPARC stack pointer requires alignment
|
||
|
|
on 8 bytes, so you cannot push a 4-byte integer on the stack
|
||
|
|
and then \*(Sisub 4, %sp\*(So\(dd.
|
||
|
|
.FS
|
||
|
|
\(dd For more information about SPARC assembler see the Sun-4 Assembly
|
||
|
|
Language Reference Manual
|
||
|
|
.FE
|
||
|
|
This too will be discussed in the next chapter, where we will take a
|
||
|
|
more in-depth look into this problem and also discuss a couple of
|
||
|
|
possible solutions.
|
||
|
|
.PP
|
||
|
|
Another thing is that \fIcc\fR usually passes the first six parameters of a
|
||
|
|
function-call through registers. To be \fI.o\fR-compatible we would have to
|
||
|
|
pass the first six parameters of each function call through registers as well.
|
||
|
|
Exactly why this is not feasible will also be discussed in the next chapter.
|
||
|
|
.bp
|