[PATCH] D89623: [MemCpyOpt] Move GEP during call slot optimization

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: efriedma, jdoerfert.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
nikic requested review of this revision.

When performing a call slot optimization to a GEP dest, it will currently usually not apply, because the GEP is directly before the memcpy and as such does not dominate the call. We should move it above the call if that satisfies the domination requirement. I think that a constant-index GEP is the only useful thing to move here, as otherwise isDereferenceablePointer couldn't look through it anyway. As such I'm not trying to generalize this further.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D89623

Files:
  llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
  llvm/test/Transforms/MemCpyOpt/callslot.ll


Index: llvm/test/Transforms/MemCpyOpt/callslot.ll
===================================================================
--- llvm/test/Transforms/MemCpyOpt/callslot.ll
+++ llvm/test/Transforms/MemCpyOpt/callslot.ll
@@ -150,9 +150,10 @@
 ; CHECK-NEXT:    [[DEST:%.*]] = alloca [16 x i8], align 1
 ; CHECK-NEXT:    [[SRC:%.*]] = alloca [8 x i8], align 1
 ; CHECK-NEXT:    [[SRC_I8:%.*]] = bitcast [8 x i8]* [[SRC]] to i8*
-; CHECK-NEXT:    call void @accept_ptr(i8* [[SRC_I8]]) [[ATTR3]]
 ; CHECK-NEXT:    [[DEST_I8:%.*]] = getelementptr [16 x i8], [16 x i8]* [[DEST]], i64 0, i64 8
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DEST_I8]], i8* [[SRC_I8]], i64 8, i1 false)
+; CHECK-NEXT:    [[DEST_I81:%.*]] = bitcast i8* [[DEST_I8]] to [8 x i8]*
+; CHECK-NEXT:    [[DEST_I812:%.*]] = bitcast [8 x i8]* [[DEST_I81]] to i8*
+; CHECK-NEXT:    call void @accept_ptr(i8* [[DEST_I812]]) [[ATTR3]]
 ; CHECK-NEXT:    ret void
 ;
   %dest = alloca [16 x i8]
Index: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -913,10 +913,20 @@
 
   // Since we're changing the parameter to the callsite, we need to make sure
   // that what would be the new parameter dominates the callsite.
-  // TODO: Support moving instructions like GEPs upwards.
-  if (Instruction *cpyDestInst = dyn_cast<Instruction>(cpyDest))
-    if (!DT->dominates(cpyDestInst, C))
+  auto DominatesCall = [this, C](Value *V) {
+    if (auto *I = dyn_cast<Instruction>(V))
+      return DT->dominates(I, C);
+    return true;
+  };
+  if (!DominatesCall(cpyDest)) {
+    // Support moving a constant index GEP before the call.
+    auto *GEP = dyn_cast<GetElementPtrInst>(cpyDest);
+    if (GEP && GEP->hasAllConstantIndices() &&
+        DominatesCall(GEP->getPointerOperand()))
+      GEP->moveBefore(C);
+    else
       return false;
+  }
 
   // In addition to knowing that the call does not access src in some
   // unexpected manner, for example via a global, which we deduce from


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D89623.298826.patch
Type: text/x-patch
Size: 2126 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20201017/0140141e/attachment.bin>