[PATCH] D51396: [Constant Hoisting] Hoisting Constant GEP Expressions

[Z. Zheng via Phabricator via llvm-commits]

zzheng marked 4 inline comments as done.
zzheng added inline comments.


================
Comment at: lib/Transforms/Scalar/ConstantHoisting.cpp:407
+  int Cost = 3;;
+  if (Cost > TTI->getIntImmCost(Instruction::Add, 1, Offset, PtrIntTy)) {
+    ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV];
----------------
efriedma wrote:
> zzheng wrote:
> > efriedma wrote:
> > > This still seems weird; we only hoist if the cost is less than 3?
> > We want to hoist when cost of hoisting is smaller than cost of not-hoisting.
> > 
> > Currently a constant GEP whose base pointer is a GV, is lowered to a CP entry. The initial value of Cost reflects cost of loading this CP entry.
> > 
> > If loading from CP is always more expensive than computing it by <base + offset>, perhaps we can get rid of this if condition?
> Making it unconditional is probably okay.
> 
> Well, we might not want to hoist if the offset is so small we aren't actually getting any benefit.  (e.g. loading from `[r0]`  and `[r0, #4]`  isn't really better than loading from `[r0, #8]` and `[r0, #12]`).
This patch tries to change from loading a group of CP entries to loading one CP entry and loading the reset by <Base + Offset>.

However, that's the case on our internal tree. We have a patch that lowers constant GEPs to CP entries


Repository:
  rL LLVM

https://reviews.llvm.org/D51396