[PATCH] D119571: [funcattrs] use DominatorTree to improve noreturn

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers created this revision.
nickdesaulniers added reviewers: jdoerfert, reames, rnk.
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There was a fixme in the code pertaining to attributing functions as noreturn.
By using dominance, 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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D119571

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


Index: llvm/test/Transforms/FunctionAttrs/noreturn.ll
===================================================================
--- llvm/test/Transforms/FunctionAttrs/noreturn.ll
+++ llvm/test/Transforms/FunctionAttrs/noreturn.ll
@@ -40,9 +40,8 @@
   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 @@
 }
 
 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)
Index: llvm/lib/Transforms/IPO/FunctionAttrs.cpp
===================================================================
--- llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -38,6 +38,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/InstrTypes.h"
@@ -1622,9 +1623,12 @@
         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)) {
+    const DominatorTree DT(*F);
+    if (none_of(*F, [&DT](BasicBlock &BB) {
+          // FIXME: this doesn't handle recursion or infinite loops.
+          return DT.isReachableFromEntry(DT.getNode(&BB)) &&
+                 basicBlockCanReturn(BB);
+        })) {
       F->setDoesNotReturn();
       Changed.insert(F);
     }


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