[PATCH] D115898: Allow calls with known writes when trying to remove allocas [part 2]

[Philip Reames via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG682b083bbd24: Allow calls with known writes when trying to remove allocas [part 2] (authored by reames).

Changed prior to commit:
  https://reviews.llvm.org/D115898?vs=394962&id=394975#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D115898/new/

https://reviews.llvm.org/D115898

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/trivial-dse-calls.ll


Index: llvm/test/Transforms/InstCombine/trivial-dse-calls.ll
===================================================================
--- llvm/test/Transforms/InstCombine/trivial-dse-calls.ll
+++ llvm/test/Transforms/InstCombine/trivial-dse-calls.ll
@@ -50,6 +50,18 @@
   ret void
 }
 
+; As long as the result is unused, we can even remove reads of the alloca
+; itself since the write will be dropped.
+define void @test_dead_readwrite() {
+; CHECK-LABEL: @test_dead_readwrite(
+; CHECK-NEXT:    ret void
+;
+  %a = alloca i32, align 4
+  %bitcast = bitcast i32* %a to i8*
+  call void @f(i8* nocapture %bitcast) argmemonly nounwind willreturn
+  ret void
+}
+
 define i32 @test_neg_read_after() {
 ; CHECK-LABEL: @test_neg_read_after(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 4
@@ -198,3 +210,15 @@
   ret void
 }
 
+; As long as the result is unused, we can even remove reads of the alloca
+; itself since the write will be dropped.
+define void @test_self_read() {
+; CHECK-LABEL: @test_self_read(
+; CHECK-NEXT:    ret void
+;
+  %a = alloca i32, align 4
+  %bitcast = bitcast i32* %a to i8*
+  call void @f2(i8* nocapture writeonly %bitcast, i8* nocapture readonly %bitcast) argmemonly nounwind willreturn
+  ret void
+}
+
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2561,7 +2561,7 @@
 }
 
 /// Given a call CB which uses an address UsedV, return true if we can prove the
-/// calls only effect is storing to V.
+/// call's only possible effect is storing to V.
 static bool isRemovableWrite(CallBase &CB, Value *UsedV) {
   if (!CB.use_empty())
     // TODO: add recursion if returned attribute is present
@@ -2580,18 +2580,15 @@
     if (!CB.doesNotCapture(i))
       // capture would allow the address to be read back in an untracked manner
       return false;
-    if (UsedV != CB.getArgOperand(i)) {
-      if (!CB.onlyReadsMemory(i))
-        // A write to another memory location keeps the call live, and thus we
-        // must keep the alloca so that the call has somewhere to write to.
-        // TODO: This results in an inprecision when two values derived from the
-        // same alloca are passed as arguments to the same function.
-        return false;
-      continue;
-    }
-    if (!CB.paramHasAttr(i, Attribute::WriteOnly))
-      // a read would hold the address live
+    if (UsedV != CB.getArgOperand(i) && !CB.onlyReadsMemory(i))
+      // A write to another memory location keeps the call live, and thus we
+      // must keep the alloca so that the call has somewhere to write to.
+      // TODO: This results in an inprecision when two values derived from the
+      // same alloca are passed as arguments to the same function.
       return false;
+    // Note: Both reads from and writes to the alloca are fine.  Since the
+    // result is unused nothing can observe the values read from the alloca
+    // without writing it to some other observable location (checked above).
   }
   return true;
 }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D115898.394975.patch
Type: text/x-patch
Size: 3159 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20211216/6c1bf6d1/attachment.bin>