[llvm] 6cd44b2 - [FunctionAttrs] Derive willreturn for fns with readonly` & `mustprogress`.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2021-01-12T20:02:34Z
New Revision: 6cd44b204c6c6f2e915270af6792f247c4c23abc

URL: https://github.com/llvm/llvm-project/commit/6cd44b204c6c6f2e915270af6792f247c4c23abc
DIFF: https://github.com/llvm/llvm-project/commit/6cd44b204c6c6f2e915270af6792f247c4c23abc.diff

LOG: [FunctionAttrs] Derive willreturn for fns with readonly` & `mustprogress`.

Similar to D94125, derive `willreturn` for functions that are `readonly` and
`mustprogress` in FunctionAttrs.

To quote the reasoning from D94125:

    Since D86233 we have `mustprogress` which, in combination with
    `readonly`, implies `willreturn`. The idea is that every side-effect
    has to be modeled as a "write". Consequently, `readonly` means there
    is no side-effect, and `mustprogress` guarantees that we cannot "loop"
    forever without side-effect.

Reviewed By: jdoerfert, nikic

Differential Revision: https://reviews.llvm.org/D94502

Added: 
    

Modified: 
    llvm/include/llvm/IR/Function.h
    llvm/lib/Transforms/IPO/FunctionAttrs.cpp
    llvm/test/Transforms/FunctionAttrs/willreturn.ll

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/IR/Function.h b/llvm/include/llvm/IR/Function.h
index 019e3a98a1af..7e209bb3769b 100644
--- a/llvm/include/llvm/IR/Function.h
+++ b/llvm/include/llvm/IR/Function.h
@@ -640,6 +640,10 @@ class Function : public GlobalObject, public ilist_node<Function> {
   }
   void setMustProgress() { addFnAttr(Attribute::MustProgress); }
 
+  /// Determine if the function will return.
+  bool willReturn() const { return hasFnAttribute(Attribute::WillReturn); }
+  void setWillReturn() { addFnAttr(Attribute::WillReturn); }
+
   /// True if the ABI mandates (or the user requested) that this
   /// function be in a unwind table.
   bool hasUWTable() const {

diff  --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 5cf5e9463b45..2e24cad1393b 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -77,6 +77,7 @@ STATISTIC(NumNonNullReturn, "Number of function returns marked nonnull");
 STATISTIC(NumNoRecurse, "Number of functions marked as norecurse");
 STATISTIC(NumNoUnwind, "Number of functions marked as nounwind");
 STATISTIC(NumNoFree, "Number of functions marked as nofree");
+STATISTIC(NumWillReturn, "Number of functions marked as willreturn");
 
 static cl::opt<bool> EnableNonnullArgPropagation(
     "enable-nonnull-arg-prop", cl::init(true), cl::Hidden,
@@ -1424,6 +1425,22 @@ static bool addNoReturnAttrs(const SCCNodeSet &SCCNodes) {
   return Changed;
 }
 
+// Set the willreturn function attribute if possible.
+static bool addWillReturn(const SCCNodeSet &SCCNodes) {
+  bool Changed = false;
+
+  for (Function *F : SCCNodes) {
+    if (!F || !F->onlyReadsMemory() || !F->mustProgress() || F->willReturn())
+      continue;
+
+    F->setWillReturn();
+    NumWillReturn++;
+    Changed = true;
+  }
+
+  return Changed;
+}
+
 static SCCNodesResult createSCCNodeSet(ArrayRef<Function *> Functions) {
   SCCNodesResult Res;
   Res.HasUnknownCall = false;
@@ -1468,6 +1485,7 @@ static bool deriveAttrsInPostOrder(ArrayRef<Function *> Functions,
   Changed |= addArgumentAttrs(Nodes.SCCNodes);
   Changed |= inferConvergent(Nodes.SCCNodes);
   Changed |= addNoReturnAttrs(Nodes.SCCNodes);
+  Changed |= addWillReturn(Nodes.SCCNodes);
 
   // If we have no external nodes participating in the SCC, we can deduce some
   // more precise attributes as well.

diff  --git a/llvm/test/Transforms/FunctionAttrs/willreturn.ll b/llvm/test/Transforms/FunctionAttrs/willreturn.ll
index 56ca12638e9c..d92151c299fe 100644
--- a/llvm/test/Transforms/FunctionAttrs/willreturn.ll
+++ b/llvm/test/Transforms/FunctionAttrs/willreturn.ll
@@ -1,9 +1,8 @@
 ; RUN: opt -function-attrs -S %s | FileCheck %s
 
-; TODO
 define void @mustprogress_readnone() mustprogress {
-; CHECK-NOT: Function Attrs: {{.*}} willreturn
-; CHECK:     define void @mustprogress_readnone()
+; CHECK:      Function Attrs: {{.*}} noreturn {{.*}} readnone willreturn
+; CHECK-NEXT: define void @mustprogress_readnone()
 ;
 entry:
   br label %while.body
@@ -12,10 +11,9 @@ while.body:
   br label %while.body
 }
 
-; TODO
 define i32 @mustprogress_load(i32* %ptr) mustprogress {
-; CHECK-NOT: Function Attrs: {{.*}} willreturn
-; CHECK:     define i32 @mustprogress_load(
+; CHECK:      Function Attrs: {{.*}} readonly willreturn
+; CHECK-NEXT: define i32 @mustprogress_load(
 ;
 entry:
   %r = load i32, i32* %ptr
@@ -35,16 +33,15 @@ declare void @unknown_fn()
 
 define void @mustprogress_call_unknown_fn() mustprogress {
 ; CHECK-NOT: Function Attrs: {{.*}} willreturn
-; CHECK: define void @mustprogress_call_unknown_fn(
+; CHECK:     define void @mustprogress_call_unknown_fn(
 ;
   call void @unknown_fn()
   ret void
 }
 
-; TODO
 define i32 @mustprogress_call_known_functions(i32* %ptr) mustprogress {
-; CHECK-NOT: Function Attrs: {{.*}} willreturn
-; CHECK:     define i32 @mustprogress_call_known_functions(
+; CHECK:      Function Attrs: {{.*}} readonly willreturn
+; CHECK-NEXT: define i32 @mustprogress_call_known_functions(
 ;
   call void @mustprogress_readnone()
   %r = call i32 @mustprogress_load(i32* %ptr)
@@ -53,10 +50,9 @@ define i32 @mustprogress_call_known_functions(i32* %ptr) mustprogress {
 
 declare i32 @__gxx_personality_v0(...)
 
-; TODO
 define i64 @mustprogress_mayunwind() mustprogress personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
-; CHECK-NOT: Function Attrs: {{.*}} willreturn
-; CHECK:     define i64 @mustprogress_mayunwind(
+; CHECK:      Function Attrs: {{.*}} readnone willreturn
+; CHECK-NEXT: define i64 @mustprogress_mayunwind(
 ;
   %a = invoke i64 @fn_noread()
           to label %A unwind label %B