[llvm-dev] [RFC] Propeller: A frame work for Post Link Optimizations

Maksim Panchenko via llvm-dev llvm-dev at lists.llvm.org
Wed Oct 2 13:24:32 PDT 2019

Thanks for clarifying. This means once you move to the next basic block (or any other basic
block in the function) you have to execute an entirely new set of CFI instructions
except for the common CIE part. While indeed this is not as bad, on average, the overall
active memory footprint will increase.

Creating one FDE per basic block means that .eh_frame_hdr, an allocatable section,
will be bloated too. This will increase the FDE lookup time. I don’t see .eh_frame_hdr
being mentioned in the proposal.


On 10/2/19, 12:20 PM, "Krzysztof Pszeniczny" <kpszeniczny at google.com<mailto:kpszeniczny at google.com>> wrote:

On Wed, Oct 2, 2019 at 8:41 PM Maksim Panchenko via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
*Pessimization/overhead for stack unwinding used by system-wide profilers and
for exception handling*

Larger CFI programs put an extra burden on unwinding at runtime as more CFI
(and thus native) instructions have to be executed. This will cause more
overhead for any profiler that records stack traces, and, as you correctly note
in the proposal, for any program that heavily uses exceptions.

The number of CFI instructions that have to be executed when unwinding any given stack stays the same. The CFI instructions for a function have to be duplicated in every basic block section, but when performing unwinding only one such a set is executed -- the copy for the current basic block. However, this copy contains precisely the same CFI instructions as the ones that would have to be executed if there were no basic block sections.

Krzysztof Pszeniczny
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