Hello all,
In r223757 I’ve committed a patch that performs, for the 32-bit x86 calling convention, the transformation of MOV instructions that push function arguments onto the stack into actual PUSH instructions.
For example, it will transform this:
subl $16, %esp
movl $4, 12(%esp)
movl $3, 8(%esp)
movl $2, 4(%esp)
movl $1, (%esp)
calll _func
addl $16, %esp
Into this:
pushl $4
pushl $3
pushl $2
pushl $1
calll _func
addl $16, %esp
The main motivation for this is code size (a “pushl $4” is 2 bytes, a “movl $4, 12(%esp)” is 7 bytes), but there are some other advantages, as shown below.
The way this works in r223757 is by intercepting call frame simplification in the Prolog/Epilog Inserter, and replacing the mov sequence with pushes. Right now it only handles functions which do not have a reserved call frame (a small minority of cases), and I’d like to extend it to cover other cases where it is profitable.
The currently implemented approach has a couple of drawbacks:
- Push vs. having a reserved call frame:
This transformation is always profitable when we do not have a reserved call frame. When a reserved frame can be used, however, there is a trade-off. For example, in a function that contains only one call site, and no other stack allocations, pushes are a clear win, since having a reserved call frame wouldn’t save any instructions. On the other hand, if a function contains 10 call sites, and only one of them can use pushes, then it is most probably a loss – not reserving a call frame will cost us 10 add instructions, and the pushes gain very little. I’d like to be able to make the decision on whether we want to have a reserved frame or pushes by considering the entire function. I don’t think this can be done in the context of PEI.
Note that in theory we could have both a reserved call frame and have some specific call sites in the function use pushes, but this is fairly tricky because it requires much more precise tracking of the stack pointer state. That is something I’m not planning to implement at this point.
- Register allocation inefficiency:
Ideally, pushes can be used to make direct memory-to-memory movs, freeing up registers, and saving quite a lot of code.
For example, for this (this is obviously a constructed example, but code of this kind does exist in the wild):
void foo(int a, int b, int c, int d, int e, int f, int g, int h);
struct st { int arr[8]; };
void bar(struct st* p)
{
foo(p->arr[0], p->arr[1], p->arr[2], p->arr[3], p->arr[4], p->arr[5], p->arr[6], p->arr[7]); }
We currently generate (with -m32 -O2) this:
pushl %ebp
movl %esp, %ebp
pushl %ebx
pushl %edi
pushl %esi
subl $44, %esp
movl 8(%ebp), %eax
movl 28(%eax), %ecx
movl %ecx, -20(%ebp) # 4-byte Spill
movl 24(%eax), %edx
movl 20(%eax), %esi
movl 16(%eax), %edi
movl 12(%eax), %ebx
movl 8(%eax), %ecx
movl %ecx, -24(%ebp) # 4-byte Spill
movl (%eax), %ecx
movl %ecx, -16(%ebp) # 4-byte Spill
movl 4(%eax), %eax
movl -20(%ebp), %ecx # 4-byte Reload
movl %ecx, 28(%esp)
movl %edx, 24(%esp)
movl %esi, 20(%esp)
movl %edi, 16(%esp)
movl %ebx, 12(%esp)
movl -24(%ebp), %ecx # 4-byte Reload
movl %ecx, 8(%esp)
movl %eax, 4(%esp)
movl -16(%ebp), %eax # 4-byte Reload
movl %eax, (%esp)
calll _foo
addl $44, %esp
popl %esi
popl %edi
popl %ebx
popl %ebp
retl
Which is fairly horrible.
Some parameters get mov-ed up to four times - a mov from the struct into a register, a register spill, a reload, and finally a mov onto the stack.
What we’d like to generate is something like:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
pushl 28(%eax)
pushl 24(%eax)
pushl 20(%eax)
pushl 16(%eax)
pushl 12(%eax)
pushl 8(%eax)
pushl 4(%eax)
pushl (%eax)
calll _foo
addl $32, %esp
popl %ebp
retl
To produce the code above, the transformation has to run pre-reg-alloc. Otherwise, even if we fold loads into the push, it’s too late to recover from spills.
The direction I’d like to take with this is:
- Add an X86-specific MachineFunctionPass that does the mov → push transformation and runs pre-reg-alloc.
It will:
-
Make a decision on whether promoting some (or all) of the call sites to use pushes is worth giving up on the reserved call frame.
-
If it is, perform the mov ->push transformation for the selected call sites.
-
Fold loads into the pushes while doing the transformation.
As an alternative, I could try to teach the peephole optimizer to do it (right now it won’t even try to do this folding because PUSHes store to memory), but getting it right in the general case seems complex.
I think I’d rather do folding of the simple (but common) cases on the fly.
- Alter the semantics of ADJCALLSTACKDOWN/ADJCALLSTACKUP slightly.
Doing the mov->push transformation before PEI means I’d have to leave the ADJCALLSTACKDOWN/UP pair unbalanced.
E.g. something like:
ADJCALLSTACKDOWN32 0, %ESP, %EFLAGS<imp-def,dead>, %ESP
%vreg9<def,dead> = COPY %ESP; GR32:%vreg9
PUSH32rmm %vreg0, 1, %noreg, 28, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 24, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 20, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 16, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 12, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 8, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 4, %noreg, %ESP, %ESP; GR32:%vreg0
PUSH32rmm %vreg0, 1, %noreg, 0, %noreg, %ESP, %ESP; GR32:%vreg0
CALLpcrel32 ga:foo, , %ESP, %ESP
ADJCALLSTACKUP32 32, 0, %ESP, %EFLAGS<imp-def,dead>, %ESP
This, rightly, gets flagged by the verifier.
My proposal is to add an additional parameter to ADJCALLSTACKDOWN to express the amount of adjustment the call sequence itself does. This is somewhat similar to the second parameter of ADKCALLSTACKUP which allows adjustment for callee stack-clean-up.
So, in this case, we will get a “ADJCALLSTACKDOWN32 32, 32” instead of the “ADJCALLSTACKDOWN32 0”. The verifier will be happy, and PEI will know it doesn’t need to do any stack pointer adjustment.
Does this sound like the right approach?
Any suggestions, as well as warnings of potential pitfalls, are welcome. 
Thanks,
Michael