From ea25b48af33be42e19236d8eac26bd42b45bcc1b Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Fri, 16 Apr 2010 15:57:50 +0000
Subject: [PATCH] Refine the detection of seemingly infinitely recursive calls
 where the callee is expected to be expanded to something else by codegen, so
 that normal infinitely recursive calls are still transformed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101468 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../Scalar/TailRecursionElimination.cpp       | 30 +++++++++++++------
 test/Transforms/TailCallElim/inf-recursion.ll | 26 +++++++++++++++-
 2 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/lib/Transforms/Scalar/TailRecursionElimination.cpp b/lib/Transforms/Scalar/TailRecursionElimination.cpp
index 667e4d9a95e..a29047c8af3 100644
--- a/lib/Transforms/Scalar/TailRecursionElimination.cpp
+++ b/lib/Transforms/Scalar/TailRecursionElimination.cpp
@@ -59,6 +59,8 @@
 #include "llvm/Instructions.h"
 #include "llvm/Pass.h"
 #include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/InlineCost.h"
+#include "llvm/Support/CallSite.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/ADT/Statistic.h"
 using namespace llvm;
@@ -328,15 +330,6 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
   if (&BB->front() == Ret) // Make sure there is something before the ret...
     return false;
   
-  // If the return is in the entry block, then making this transformation would
-  // turn infinite recursion into an infinite loop.  This transformation is ok
-  // in theory, but breaks some code like:
-  //   double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
-  // disable this xform in this case, because the code generator will lower the
-  // call to fabs into inline code.
-  if (BB == &F->getEntryBlock())
-    return false;
-
   // Scan backwards from the return, checking to see if there is a tail call in
   // this block.  If so, set CI to it.
   CallInst *CI;
@@ -356,6 +349,25 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
   if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
     return false;
 
+  // As a special case, detect code like this:
+  //   double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
+  // and disable this xform in this case, because the code generator will
+  // lower the call to fabs into inline code.
+  if (BB == &F->getEntryBlock() && 
+      &BB->front() == CI && &*++BB->begin() == Ret &&
+      callIsSmall(F)) {
+    // A single-block function with just a call and a return. Check that
+    // the arguments match.
+    CallSite::arg_iterator I = CallSite(CI).arg_begin(),
+                           E = CallSite(CI).arg_end();
+    Function::arg_iterator FI = F->arg_begin(),
+                           FE = F->arg_end();
+    for (; I != E && FI != FE; ++I, ++FI)
+      if (*I != &*FI) break;
+    if (I == E && FI == FE)
+      return false;
+  }
+
   // If we are introducing accumulator recursion to eliminate associative
   // operations after the call instruction, this variable contains the initial
   // value for the accumulator.  If this value is set, we actually perform
diff --git a/test/Transforms/TailCallElim/inf-recursion.ll b/test/Transforms/TailCallElim/inf-recursion.ll
index a5f246d36ce..e4ac9283aec 100644
--- a/test/Transforms/TailCallElim/inf-recursion.ll
+++ b/test/Transforms/TailCallElim/inf-recursion.ll
@@ -1,6 +1,10 @@
-; RUN: opt < %s -tailcallelim -S | grep call
+; RUN: opt < %s -tailcallelim -S | FileCheck %s
+
 ; Don't turn this into an infinite loop, this is probably the implementation
 ; of fabs and we expect the codegen to lower fabs.
+; CHECK: @fabs(double %f)
+; CHECK: call
+; CHECK: ret
 
 define double @fabs(double %f) {
 entry:
@@ -8,3 +12,23 @@ entry:
         ret double %tmp2
 }
 
+; Do turn other calls into infinite loops though.
+
+; CHECK: define double @foo
+; CHECK-NOT: call
+; CHECK: }
+define double @foo(double %f) {
+        %t= call double @foo(double %f)
+        ret double %t
+}
+
+; CHECK: define float @fabsf
+; CHECK-NOT: call
+; CHECK: }
+define float @fabsf(float %f) {
+        %t= call float @fabsf(float 2.0)
+        ret float %t
+}
+
+declare float @fabsf(float %f)
+declare x86_fp80 @fabsl(x86_fp80 %f)