[llvm] 903c3d2 - [SCEVExpander] Always use i8 GEP for reused value offset

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2022-01-26T15:38:58+01:00
New Revision: 903c3d2863b9d0ae760a2f45e0e934f1d12cd1d4

URL: https://github.com/llvm/llvm-project/commit/903c3d2863b9d0ae760a2f45e0e934f1d12cd1d4
DIFF: https://github.com/llvm/llvm-project/commit/903c3d2863b9d0ae760a2f45e0e934f1d12cd1d4.diff

LOG: [SCEVExpander] Always use i8 GEP for reused value offset

We could keep the non-i8 GEP code for non-opaque pointers, but
there's two reasons I'm dropping it: First, this actually appears
to be dead code, at least it isn't hit in any of our tests. I
expect that this is because we usually expand trip counts, and
those are never pointers (anymore). Second, the non-i8 GEP was
actually incorrect in multiple ways, because it used SCEV type
sizes, which don't match DL type sizes (for pointers) and certainly
don't match type alloc sizes (which is what GEPs actually use).
As such, I'm simplifying the code to always use the i8 GEP code
path if it does get hit.

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 1495ea1a4088..5363a851fc27 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1981,22 +1981,14 @@ Value *SCEVExpander::expand(const SCEV *S) {
 
     if (VO.second) {
       if (PointerType *Vty = dyn_cast<PointerType>(V->getType())) {
-        Type *Ety = Vty->getPointerElementType();
         int64_t Offset = VO.second->getSExtValue();
-        int64_t ESize = SE.getTypeSizeInBits(Ety);
-        if ((Offset * 8) % ESize == 0) {
-          ConstantInt *Idx =
-            ConstantInt::getSigned(VO.second->getType(), -(Offset * 8) / ESize);
-          V = Builder.CreateGEP(Ety, V, Idx, "scevgep");
-        } else {
-          ConstantInt *Idx =
-            ConstantInt::getSigned(VO.second->getType(), -Offset);
-          unsigned AS = Vty->getAddressSpace();
-          V = Builder.CreateBitCast(V, Type::getInt8PtrTy(SE.getContext(), AS));
-          V = Builder.CreateGEP(Type::getInt8Ty(SE.getContext()), V, Idx,
-                                "uglygep");
-          V = Builder.CreateBitCast(V, Vty);
-        }
+        ConstantInt *Idx =
+          ConstantInt::getSigned(VO.second->getType(), -Offset);
+        unsigned AS = Vty->getAddressSpace();
+        V = Builder.CreateBitCast(V, Type::getInt8PtrTy(SE.getContext(), AS));
+        V = Builder.CreateGEP(Type::getInt8Ty(SE.getContext()), V, Idx,
+                              "uglygep");
+        V = Builder.CreateBitCast(V, Vty);
       } else {
         V = Builder.CreateSub(V, VO.second);
       }