[llvm] 9dcb006 - [funcattrs] check reachability to improve noreturn

[Nick Desaulniers via llvm-commits]

Author: Nick Desaulniers
Date: 2022-02-14T14:01:59-08:00
New Revision: 9dcb0061657e9b7f321fa6c295960c8f829ed6f1

URL: https://github.com/llvm/llvm-project/commit/9dcb0061657e9b7f321fa6c295960c8f829ed6f1
DIFF: https://github.com/llvm/llvm-project/commit/9dcb0061657e9b7f321fa6c295960c8f829ed6f1.diff

LOG: [funcattrs] check reachability to improve noreturn

There was a fixme in the code pertaining to attributing functions as
noreturn.  By using reachability, if none of the blocks that are
reachable from the entry return, then the function is noreturn.

Previously, the code only checked if any blocks returned. If they're
unreachable, then they don't matter.

This improves codegen for the Linux kernel.

Fixes: https://github.com/ClangBuiltLinux/linux/issues/1563

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D119571

Added: 
    

Modified: 
    llvm/lib/Transforms/IPO/FunctionAttrs.cpp
    llvm/test/Transforms/FunctionAttrs/noreturn.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 213a998d5bba2..e2f1944cee63f 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -1614,6 +1614,26 @@ static bool basicBlockCanReturn(BasicBlock &BB) {
   return none_of(BB, instructionDoesNotReturn);
 }
 
+// FIXME: this doesn't handle recursion.
+static bool canReturn(Function &F) {
+  SmallVector<BasicBlock *, 16> Worklist;
+  SmallPtrSet<BasicBlock *, 16> Visited;
+
+  Visited.insert(&F.front());
+  Worklist.push_back(&F.front());
+
+  do {
+    BasicBlock *BB = Worklist.pop_back_val();
+    if (basicBlockCanReturn(*BB))
+      return true;
+    for (BasicBlock *Succ : successors(BB))
+      if (Visited.insert(Succ).second)
+        Worklist.push_back(Succ);
+  } while (!Worklist.empty());
+
+  return false;
+}
+
 // Set the noreturn function attribute if possible.
 static void addNoReturnAttrs(const SCCNodeSet &SCCNodes,
                              SmallSet<Function *, 8> &Changed) {
@@ -1622,9 +1642,7 @@ static void addNoReturnAttrs(const SCCNodeSet &SCCNodes,
         F->doesNotReturn())
       continue;
 
-    // The function can return if any basic blocks can return.
-    // FIXME: this doesn't handle recursion or unreachable blocks.
-    if (none_of(*F, basicBlockCanReturn)) {
+    if (!canReturn(*F)) {
       F->setDoesNotReturn();
       Changed.insert(F);
     }

diff  --git a/llvm/test/Transforms/FunctionAttrs/noreturn.ll b/llvm/test/Transforms/FunctionAttrs/noreturn.ll
index eba56c9630adb..6bc1e32ed5165 100644
--- a/llvm/test/Transforms/FunctionAttrs/noreturn.ll
+++ b/llvm/test/Transforms/FunctionAttrs/noreturn.ll
@@ -40,9 +40,8 @@ end:
   ret i32 %c
 }
 
-; CHECK-NOT: Function Attrs: {{.*}}noreturn
+; CHECK: Function Attrs: {{.*}}noreturn
 ; CHECK: @caller5()
-; We currently don't handle unreachable blocks.
 define i32 @caller5() {
 entry:
   %c = call i32 @noreturn()
@@ -87,4 +86,4 @@ define void @coro() "coroutine.presplit"="1" {
 }
 
 declare token @llvm.coro.id.retcon.once(i32 %size, i32 %align, i8* %buffer, i8* %prototype, i8* %alloc, i8* %free)
-declare i1 @llvm.coro.end(i8*, i1)
\ No newline at end of file
+declare i1 @llvm.coro.end(i8*, i1)