[PATCH] D90688: [CaptureTracking] Avoid overly restrictive dominates check

[Anna Thomas via Phabricator via llvm-commits]

anna created this revision.
anna added reviewers: jdoerfert, apilipenko, reames, hfinkel.
Herald added subscribers: dantrushin, hiraditya.
Herald added a project: LLVM.
anna requested review of this revision.

CapturesBefore tracker has an overly restrictive dominates check when
the `BeforeHere` and the capture point are in different basic blocks.
All we need to check is that there is no path from the capture point
to `BeforeHere` (which is less stricter than the dominates check).
See added testcase in one of the users of CapturesBefore.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D90688

Files:
  llvm/lib/Analysis/CaptureTracking.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
@@ -186,6 +186,39 @@
   ret void
 }
 
+; There is no path from the capture back to the memcpy.
+; So we can perform the call slot optimization.
+define void @capture_nopath_call_argmemonly(i1 %cond) {
+; CHECK-LABEL: @capture_nopath_call_argmemonly(
+; CHECK-NEXT:    [[DEST:%.*]] = alloca [16 x i8], align 1
+; CHECK-NEXT:    [[SRC:%.*]] = alloca [16 x i8], align 1
+; CHECK-NEXT:    [[DEST_I8:%.*]] = bitcast [16 x i8]* [[DEST]] to i8*
+; CHECK-NEXT:    [[SRC_I8:%.*]] = bitcast [16 x i8]* [[SRC]] to i8*
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[CAPTURES:%.*]], label [[NOCAPTURES:%.*]]
+; CHECK:       captures:
+; CHECK-NEXT:    call void @accept_ptr(i8* [[DEST_I8]])
+; CHECK-NEXT:    ret void
+; CHECK:       nocaptures:
+; CHECK-NEXT:    [[DEST1:%.*]] = bitcast [16 x i8]* [[DEST]] to i8*
+; CHECK-NEXT:    call void @accept_ptr(i8* [[DEST1]]) [[ATTR5:#.*]]
+; CHECK-NEXT:    ret void
+;
+  %dest = alloca [16 x i8]
+  %src = alloca [16 x i8]
+  %dest.i8 = bitcast [16 x i8]* %dest to i8*
+  %src.i8 = bitcast [16 x i8]* %src to i8*
+  br i1 %cond, label %captures, label %nocaptures
+
+captures:
+  call void @accept_ptr(i8* %dest.i8) ; capture
+  ret void
+
+nocaptures:
+  call void @accept_ptr(i8* %src.i8) argmemonly nounwind
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dest.i8, i8* %src.i8, i64 16, i1 false)
+  ret void
+}
+
 define void @capture_before_call_argmemonly_nounwind() {
 ; CHECK-LABEL: @capture_before_call_argmemonly_nounwind(
 ; CHECK-NEXT:    [[DEST:%.*]] = alloca [16 x i8], align 1
Index: llvm/lib/Analysis/CaptureTracking.cpp
===================================================================
--- llvm/lib/Analysis/CaptureTracking.cpp
+++ llvm/lib/Analysis/CaptureTracking.cpp
@@ -133,10 +133,10 @@
         return !isPotentiallyReachableFromMany(Worklist, BB, nullptr, DT);
       }
 
-      // If the value is defined in the same basic block as use and BeforeHere,
-      // there is no need to explore the use if BeforeHere dominates use.
-      // Check whether there is a path from I to BeforeHere.
-      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
+      // If the value is defined in a different basic block than BeforeHere,
+      // there is no need to explore the use if there is no path from I to
+      // BeforeHere.
+      if (BeforeHere != I &&
           !isPotentiallyReachable(I, BeforeHere, nullptr, DT))
         return true;
 


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