Teach TailRecursionElimination to handle certain cases of nocapture escaping allocas.

Without the changes introduced into this patch, if TRE saw any allocas at all,
TRE would not perform TRE *or* mark callsites with the tail marker.

Because TRE runs after mem2reg, this inadequacy is not a death sentence. But
given a callsite A without escaping alloca argument, A may not be able to have
the tail marker placed on it due to a separate callsite B having a write-back
parameter passed in via an argument with the nocapture attribute.

Assume that B is the only other callsite besides A and B only has nocapture
escaping alloca arguments (*NOTE* B may have other arguments that are not passed
allocas). In this case not marking A with the tail marker is unnecessarily
conservative since:

  1. By assumption A has no escaping alloca arguments itself so it can not
     access the caller's stack via its arguments.

  2. Since all of B's escaping alloca arguments are passed as parameters with
     the nocapture attribute, we know that B does not stash said escaping
     allocas in a manner that outlives B itself and thus could be accessed
     indirectly by A.

With the changes introduced by this patch:

  1. If we see any escaping allocas passed as a capturing argument, we do
     nothing and bail early.

  2. If we do not see any escaping allocas passed as captured arguments but we
     do see escaping allocas passed as nocapture arguments:

       i. We do not perform TRE to avoid PR962 since the code generator produces
          significantly worse code for the dynamic allocas that would be created
          by the TRE algorithm.

       ii. If we do not return twice, mark call sites without escaping allocas
           with the tail marker. *NOTE* This excludes functions with escaping
           nocapture allocas.

  3. If we do not see any escaping allocas at all (whether captured or not):

       i. If we do not have usage of setjmp, mark all callsites with the tail
          marker.

       ii. If there are no dynamic/variable sized allocas in the function,
           attempt to perform TRE on all callsites in the function.

Based off of a patch by Nick Lewycky.

rdar://14324281.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186057 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 86 insertions, 25 deletions

diff --git a/test/Transforms/TailCallElim/basic.ll b/test/Transforms/TailCallElim/basic.ll
index 1db0e76..ffb7570 100644
--- a/test/Transforms/TailCallElim/basic.ll
+++ b/test/Transforms/TailCallElim/basic.ll
@@ -2,6 +2,8 @@
 
 declare void @noarg()
 declare void @use(i32*)
+declare void @use_nocapture(i32* nocapture)
+declare void @use2_nocapture(i32* nocapture, i32* nocapture)
 
 ; Trivial case. Mark @noarg with tail call.
 define void @test0() {
@@ -57,3 +59,87 @@ return:		; preds = %entry
 	ret i32 0
 }
 
+; Make sure that a nocapture pointer does not stop adding a tail call marker to
+; an unrelated call and additionally that we do not mark the nocapture call with
+; a tail call.
+;
+; rdar://14324281
+define void @test4() {
+; CHECK: void @test4
+; CHECK-NOT: tail call void @use_nocapture
+; CHECK: tail call void @noarg()
+; CHECK: ret void
+  %a = alloca i32
+  call void @use_nocapture(i32* %a)
+  call void @noarg()
+  ret void
+}
+
+; Make sure that we do not perform TRE even with a nocapture use. This is due to
+; bad codegen caused by PR962.
+;
+; rdar://14324281.
+define i32* @test5(i32* nocapture %A, i1 %cond) {
+; CHECK: i32* @test5
+; CHECK-NOT: tailrecurse:
+; CHECK: ret i32* null
+  %B = alloca i32
+  br i1 %cond, label %cond_true, label %cond_false
+cond_true:
+  call i32* @test5(i32* %B, i1 false)
+  ret i32* null
+cond_false:
+  call void @use2_nocapture(i32* %A, i32* %B)
+  call void @noarg()
+  ret i32* null
+}
+
+; PR14143: Make sure that we do not mark functions with nocapture allocas with tail.
+;
+; rdar://14324281.
+define void @test6(i32* %a, i32* %b) {
+; CHECK: @test6
+; CHECK-NOT: tail call
+; CHECK: ret void
+  %c = alloca [100 x i8], align 16
+  %tmp = bitcast [100 x i8]* %c to i32*
+  call void @use2_nocapture(i32* %b, i32* %tmp)
+  ret void
+}
+
+; PR14143: Make sure that we do not mark functions with nocapture allocas with tail.
+;
+; rdar://14324281
+define void @test7(i32* %a, i32* %b) nounwind uwtable {
+entry:
+; CHECK: @test7
+; CHECK-NOT: tail call
+; CHECK: ret void
+  %c = alloca [100 x i8], align 16
+  %0 = bitcast [100 x i8]* %c to i32*
+  call void @use2_nocapture(i32* %0, i32* %a)
+  call void @use2_nocapture(i32* %b, i32* %0)
+  ret void
+}
+
+; If we have a mix of escaping captured/non-captured allocas, ensure that we do
+; not do anything including marking callsites with the tail call marker.
+;
+; rdar://14324281.
+define i32* @test8(i32* nocapture %A, i1 %cond) {
+; CHECK: i32* @test8
+; CHECK-NOT: tailrecurse:
+; CHECK-NOT: tail call
+; CHECK: ret i32* null
+  %B = alloca i32
+  %B2 = alloca i32
+  br i1 %cond, label %cond_true, label %cond_false
+cond_true:
+  call void @use(i32* %B2)
+  call i32* @test8(i32* %B, i1 false)
+  ret i32* null
+cond_false:
+  call void @use2_nocapture(i32* %A, i32* %B)
+  call void @noarg()
+  ret i32* null
+}
diff --git a/test/Transforms/TailCallElim/nocapture.ll b/test/Transforms/TailCallElim/nocapture.ll
deleted file mode 100644
index e49d87c..0000000
--- a/test/Transforms/TailCallElim/nocapture.ll
+++ /dev/null
@@ -1,25 +0,0 @@
-; RUN: opt -tailcallelim -S < %s | FileCheck %s
-; XFAIL: *
-
-declare void @use(i8* nocapture, i8* nocapture)
-
-define i8* @foo(i8* nocapture %A, i1 %cond) {
-; CHECK: tailrecurse:
-; CHECK: %A.tr = phi i8* [ %A, %0 ], [ %B, %cond_true ]
-; CHECK: %cond.tr = phi i1 [ %cond, %0 ], [ false, %cond_true ]
-  %B = alloca i8
-; CHECK: %B = alloca i8
-  br i1 %cond, label %cond_true, label %cond_false
-; CHECK: br i1 %cond.tr, label %cond_true, label %cond_false
-cond_true:
-; CHECK: cond_true:
-; CHECK: br label %tailrecurse
-  call i8* @foo(i8* %B, i1 false)
-  ret i8* null
-cond_false:
-; CHECK: cond_false
-  call void @use(i8* %A, i8* %B)
-; CHECK: tail call void @use(i8* %A.tr, i8* %B)
-  ret i8* null
-; CHECK: ret i8* null
-}