[LLVMdev] Saving one part of a register pair in the callee-saved list.

[Jim Grosbach]

Hi Borja,

On Jul 10, 2012, at 6:26 PM, Borja Ferrer wrote:

> Hello, 
> 
> I would like to know if there's a way of setting the callee-saved register list inside getCalleeSavedRegs() to make the PEI pass save/restore only one half of a register pair if the other half is not being used, instead of saving the whole pair. Here is an example of what I try to explain to make things more clear:
> 
> Suppose this situation where we have a register file of 8bit regs, and that you can form regpairs by joining two adjacent regs. Pairs are pseudo regs that are formed by two 8 bit subregisters, with their lo and hi parts defined in the register.td file. Both 8 and 16bit types are legal.
> In the following function: char foo(char a, char b, char c, int d)
> Arguments are passed this way: a=R10, b=R8, c=R6, d=R5:R4, so first three come in a single reg while the last argument comes in a pair. Since all registers are callee clobbered they have to be saved/restored in the function. The current situation is that I've listed in the the callee-saved register list the whole pairs of registers instead of their parts, so I'm getting saved and restored R11:R10, R9:R8, R7:R6 and R5:R4 which is expected. However, what I would like is to not save the first 3 pairs, but only their low register parts, since the high register part is not being used (push R10+push R8+push R6+push R5:R4).
> 
> I've tried listing both the pairs and their parts, but if a pair and one of it's parts are used separately then I get both the pair and the part saved (ie: push R1:R0+push R1+push R0) duplicating code. Also, only listing the 8bit regs works BUT I'm getting machine verifier errors because registers are appearing twice in the live-ins list so I'm getting an error like this:
> *** Bad machine code: Using an undefined physical register ***
> 
> I would like to know if it's possible to handle this situation.
> 

Yes, it's definitely possible. From your description it sounds like the sub-register relationships may not be defined correctly. ARM and X86 both have examples of that. For ARM, look at the definitions of the QPR, DPR and SPR registers and associated register classes.

-Jim
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