[PATCH] D153098: [AArch64] Emit fewer CFI instructions for synchronous unwind tables

[Shoaib Meenai via Phabricator via llvm-commits]

smeenai added inline comments.


================
Comment at: llvm/test/CodeGen/AArch64/addsub-constant-folding.ll:363
 ; CHECK-NEXT:    sub sp, sp, #32
-; CHECK-NEXT:    .cfi_def_cfa_offset 32
 ; CHECK-NEXT:    str x30, [sp, #16] // 8-byte Folded Spill
+; CHECK-NEXT:    .cfi_def_cfa_offset 32
----------------
MaskRay wrote:
> smeenai wrote:
> > ikudrin wrote:
> > > MaskRay wrote:
> > > > ikudrin wrote:
> > > > > MaskRay wrote:
> > > > > > `.cfi_def_cfa_offset 32` move like this seems unnecessary. It degrades precision without reducing the number of CFI instructions for sync.
> > > > > The goal is to reduce the size of the unwind table, not just the number of CFI instructions. Having only one chunk removes `DWA_CFA_advance_loc` instructions that would otherwise be required.
> > > > Yes, the goal is fine. However, when the number of instructions does not change, it would be better not to degrade the precision by moving `.cfi_def_cfa_offset`...
> > > In this test, there is a `.cfi_offset w30, -16` instruction below. If we kept `.cfi_def_cfa_offset 32` there it was, the size of the unwind table would be bigger. Yes, only by two bytes, but still.
> > Those bytes do add up ... I measured this change to be a 72 KiB size reduction for the Facebook for Android app, which is quite nice.
> I wonder whether we just need a mode to omit most of the registers? Many registers are probably not useful for some use cases, e.g. profiling.
That would make that mode unusable for exception handling, right? My interest here is having smaller unwind tables that can still be used for exceptions.


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https://reviews.llvm.org/D153098