[LLVMdev] should PEI::calculateFrameObjectOffsets align the stack?

Rafael EspĂ­ndola rafael.espindola at gmail.com
Sat Oct 7 17:35:02 PDT 2006

> That is irrelevant.  The PEI code needs to know how much stack space is
> required for each call to allocate and align the stack frame properly.  It
> gets this info from the ADJCALLSTACK instructions.
I see. Looking at PEI::calculateCalleeSavedRegisters shows that the
ADJCALLSTACK instructions is used to set up MaxCallFrameSize. Adding
debug prints also show that in the example code the ADJCALLSTACK
argument is 4. I believe that this is the correct argument for only a
32bit integer is passed on the stack.

The problem is that MaxCallFrameSize is **not** used in
PEI::calculateFrameObjectOffsets and it must then be used in the
target specific code. The only architecture that doesn't adds
MaxCallFrameSize to the value returned by getStackSize is Sparc. This
is so because the Sparc backend generates add/sub pairs around call

> Further, you *do* need to have a frame pointer if the function contains
> any dynamic allocas.  In that case, the SP isn't fixed within a function.
> You may not care about this right now, but it is required to support C99
> VLA's and the 'alloca' C function.
Sure. Currently I am implementing the next missing feature that causes
a gcc bootstrap to fail. It shouldn't be long before I hit a VLA :-)

> -Chris


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