[PATCH] D138708: [SROA] Assert the AllocSize of i8 to be 1

[Jannik Silvanus via Phabricator via llvm-commits]

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When SROA rewrites GEPs, it tries to generate "natural"
GEPs that apply to the underlying type of the alloca.
If that fails, SROA falls back to using an i8-based GEP
with the byte-offset as index.

This however requires the AllocSize of i8 to be 1, which
is not the case with nontrivial alignment requirements,
which may e.g. occur with DXIL.
Add assertions checking this assumption.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D138708

Files:
  llvm/lib/Transforms/Scalar/SROA.cpp


Index: llvm/lib/Transforms/Scalar/SROA.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SROA.cpp
+++ llvm/lib/Transforms/Scalar/SROA.cpp
@@ -1549,9 +1549,13 @@
                              const Twine &NamePrefix) {
   // Create i8 GEP for opaque pointers.
   if (Ptr->getType()->isOpaquePointerTy()) {
-    if (Offset != 0)
-      Ptr = IRB.CreateInBoundsGEP(IRB.getInt8Ty(), Ptr, IRB.getInt(Offset),
+    if (Offset != 0) {
+      Type *Int8Ty = IRB.getInt8Ty();
+      // This may fail due to alignment
+      assert(DL.getTypeAllocSize(Int8Ty) == 1 && "Unexpected I8 size!");
+      Ptr = IRB.CreateInBoundsGEP(Int8Ty, Ptr, IRB.getInt(Offset),
                                   NamePrefix + "sroa_idx");
+    }
     return IRB.CreatePointerBitCastOrAddrSpaceCast(Ptr, PointerTy,
                                                    NamePrefix + "sroa_cast");
   }
@@ -1640,11 +1644,14 @@
       Int8PtrOffset = Offset;
     }
 
-    OffsetPtr = Int8PtrOffset == 0
-                    ? Int8Ptr
-                    : IRB.CreateInBoundsGEP(IRB.getInt8Ty(), Int8Ptr,
-                                            IRB.getInt(Int8PtrOffset),
-                                            NamePrefix + "sroa_raw_idx");
+    Type *Int8Ty = IRB.getInt8Ty();
+    // This may fail due to alignment
+    assert(DL.getTypeAllocSize(Int8Ty) == 1 && "Unexpected I8 size!");
+    OffsetPtr =
+        Int8PtrOffset == 0
+            ? Int8Ptr
+            : IRB.CreateInBoundsGEP(Int8Ty, Int8Ptr, IRB.getInt(Int8PtrOffset),
+                                    NamePrefix + "sroa_raw_idx");
   }
   Ptr = OffsetPtr;
 


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