[PATCH] Rewrite calculateDbgValueHistory to make it (hopefully) more transparent.

[Paul Robinson]

> -----Original Message-----
> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
> bounces at cs.uiuc.edu] On Behalf Of Alexey Samsonov
> 
> Hi dblaikie,
> 
> This change preserves the original algorithm of generating history
> for user variables, but makes it more clear.
> 
> High-level description of algorithm:
> Scan all the machine basic blocks and machine instructions in the order
> they are emitted to the object file. Do the following:
> 1) If we see a DBG_VALUE instruction, add it to the history of the
> corresponding user variable. Keep track of all user variables, whose
> locations are described by a register.
> 2) If we see a regular instruction, look at all the registers it
> clobbers,
> and terminate the location range for all variables described by these
> registers.
> 3) At the end of the basic block, terminate location ranges for all
> user variables described by some register.
> 
> (3) is too restrictive, and may result in a poor debug info.
> For example, the variable location can be described by %rsp, or any
> other
> register which is never clobbered in a function. I think that handling
> this special case may improve situation for many real-life programs and
> plan to address this in future commits.

Taking no stand on the patch, just on that last statement...

(3) is correctly conservative, because variable-in-register liveness 
depends on control flow but debug-value ranges are expressed in terms 
of physical order.  Getting this truly right requires a dataflow 
analysis to track liveness across block boundaries.

There may be special cases of small multi-block functions where some 
variable lives in a register that is never clobbered, but I question 
how often that will come up in real programs.

If a variable has a "home" on the stack, that's a different story.
Naively you can fall back to the stack location after a register
copy is clobbered and probably 90%-95% of the time you're fine.
--paulr

> 
> http://reviews.llvm.org/D3597
> 
> Files:
>   lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp

http://reviews.llvm.org/D3597