[llvm-dev] Finding caller-saved registers at a function call site

Robert Lyerly via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 27 12:37:08 PDT 2016

Ah, I see -- the registers left out of the mask are considered clobbered.
Got it!

At a high level, I'm interested in finding the locations of all values that
are live at a given call site.  You can think of it like a debugger, e.g.
gdb -- I'd like to be able to unwind the stack, frame by frame, and locate
all the live values for each function invocation (i.e., where they are in a
function's stack frame/register set) at the function call site.  I'm
currently using DWARF metadata to implement the frame unwinding mechanism,
similarly to gdb.  I can't use DWARF metadata for live value location
information, however, because it only generates location information for
named source code variables.  I also need to locate compiler-generated
temporaries, hence I've been looking at the StackMap intrinsic [1] to
provide live value location information.  It does most of what I need, but
it does not tell me where live values stored in registers are spilled to
the stack as part of the function call procedure (whether they be in
callee- or caller-saved registers) -- it simply tells me which registers
they are stored in before/after the function call procedure.  That's the
impetus for my question.

This is *not* a problem for callee-saved registers -- these registers are
restored from the stack as part of the call frame unwinding procedure
detailed in the DWARF standard [2].  However, I'm left trying to find the
locations of the live values that were in caller-saved registers and were
spilled to the stack as part of the function call procedure (probably
during instruction selection/register allocation, I'm not familiar enough
with this process).  I realize that for a MachineInstr for a given call
there are no live values in caller-saved registers (as they would be
clobbered and lost), but where on the stack were they saved?

In a nutshell, I'm trying to figure out where values that couldn't be
placed in callee-saved registers (and that were allocated to caller-saved
registers) were spilled to the stack as part of the function call
procedure.  Hopefully this clarifies things -- thanks!

[1] http://llvm.org/docs/StackMaps.html
[2] http://dwarfstd.org/doc/DWARF4.pdf, page 140

On Mon, Jun 27, 2016 at 10:28 AM, Rob Lyerly <rlyerly at vt.edu> wrote:

> Hi Sanjoy,
> I'm having trouble finding caller-saved registers using the RegMask
> operand you've mentioned.  As an example, I've got a C function that looks
> like this:
> double recurse(int depth, double val)
> {
>     if(depth < max_depth) return recurse(depth + 1, val * 1.2) + val;
>     else return outer_func(val);
> }
> As a quick refresher, all "xmm" registers are considered caller-saved on
> x86, hence values stored in these registers should be spilled to the stack
> before a function call.  The generated assembly for branch containing the
> call to "recurse" with clang/LLVM 3.8 (-O3) on Ubuntu 14.04 looks like this:
> ...
> 400694:       ff c7                   inc    %edi                   # Add
> 1 to depth
> 400696:       f2 0f 10 05 a2 92 05    movsd  0x592a2(%rip),%xmm0    # Move
> constant 1.2 into xmm0
> 40069d:       00
> 40069e:       f2 0f 59 c1             mulsd  %xmm1,%xmm0            # val
> * 1.2
> 4006a2:       f2 0f 11 4d f8          movsd  %xmm1,-0x8(%rbp)       #
> Spill val to the stack
> 4006a7:       e8 d4 ff ff ff          callq  400680 <recurse>
> 4006ac:       f2 0f 58 45 f8          addsd  -0x8(%rbp),%xmm0       #
> recurse's return value + val
> 4006b1:       48 83 c4 10             add    $0x10,%rsp
> 4006b5:       5d                      pop    %rbp
> 4006b6:       c3                      retq
> ...
> Notice how xmm1 (the storage location of "val", which is live across the
> call to recurse) is saved onto the stack at an offset of -8 from the base
> pointer.  After the call, "val" (i.e., storage location rbp - 0x8) is used
> in the addition to calculate the returned value.  However, when I print the
> RegMask operand for the call machine instruction, I get the following:
> <regmask %BH %BL %BP %BPL %BX %EBP %EBX %RBP %RBX %R12 %R13 %R14 %R15
> %R12B %R13B %R14B %R15B %R12D %R13D %R14D %R15D %R12W %R13W %R14W %R15W>
> I don't see xmm1 as being preserved across this call.  Am I missing
> something?  Thanks for your help!
> On Wed, Jun 22, 2016 at 5:01 PM, Sanjoy Das <
> sanjoy at playingwithpointers.com> wrote:
>> Hi Rob,
>> Rob Lyerly via llvm-dev wrote:
>> > I'm looking for a way to get all the caller-saved registers (both the
>> > register and the stack slot at which it was saved) for a given function
>> > call site in the backend.  What's the best way to grab this
>> > information?  Is it possible to get this information if I have the
>> > MachineInstr of the function call?  I'm currently targeting the AArch64
>> > & X86 backends.
>> You should be able to use the RegMask operand to the MachineInstr to
>> discover the registers that are preserved or clobbered by the call
>> according to the calling convention.  For reference, you might want to
>> look at `getRegMask` and `gatherMaximalPreservedRegisters` in
>> http://reviews.llvm.org/D21115.
>> As far as discovering the slot to which it is spilled, I have no idea.
>> CC'ing Matthias for this.
>> -- Sanjoy
> --
> Rob Lyerly
> Graduate Research Assistant, Systems Software Research Group
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