indvars: Linear function test replace should avoid reusing undef.

Fixes PR13371: indvars pass incorrectly substitutes 'undef' values.

I do not like this fix. It's needed until/unless the meaning of undef
changes. It attempts to be complete according to the IR spec, but I
don't have much confidence in the implementation given the difficulty
testing undefined behavior. Worse, this invalidates some of my
hard-fought work on indvars and LSR to optimize pointer induction
variables. It results benchmark regressions, which I'll track
internally. On x86_64 no LTO I see:

-3% huffbench
-3% 400.perlbench
-8% fhourstones

My only suggestion for recovering is to change the meaning of
undef. If we could trust an arbitrary instruction to produce a some
real value that can be manipulated (e.g. incremented) according to
non-undef rules, then this case could be easily handled with SCEV.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160421 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 29 insertions, 4 deletions

diff --git a/test/Transforms/IndVarSimplify/2012-07-17-lftr-undef.ll b/test/Transforms/IndVarSimplify/2012-07-17-lftr-undef.ll
new file mode 100644
index 0000000..7c5f818
--- /dev/null
+++ b/test/Transforms/IndVarSimplify/2012-07-17-lftr-undef.ll
@@ -0,0 +1,22 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+; PR13371: indvars pass incorrectly substitutes 'undef' values
+;
+; LFTR should not user %undef as the loop counter.
+; CHECK: @test
+; CHECK-NOT: icmp{{.*}}undef
+@.str3 = private constant [6 x i8] c"%lld\0A\00", align 1
+declare i32 @printf(i8* noalias nocapture, ...) nounwind
+define i64 @test() nounwind {
+func_start:
+  br label %block9
+block9:                                           ; preds = %block9,%func_start
+  %undef = phi i64 [ %next_undef, %block9 ], [ undef, %func_start ]
+  %iter = phi i64 [ %next_iter, %block9 ], [ 1, %func_start ]
+  %next_iter = add nsw i64 %iter, 1
+  %0 = tail call i32 (i8*, ...)* @printf(i8* noalias nocapture getelementptr inbounds ([6 x i8]* @.str3, i64 0, i64 0), i64 %next_iter, i64 %undef)
+  %next_undef = add nsw i64 %undef, 1
+  %_tmp_3 = icmp slt i64 %next_iter, 100
+  br i1 %_tmp_3, label %block9, label %exit
+exit:                                             ; preds = %block9
+  ret i64 0
+}
diff --git a/test/Transforms/IndVarSimplify/lftr-reuse.ll b/test/Transforms/IndVarSimplify/lftr-reuse.ll
index 9abfe13..7fb36e5 100644
--- a/test/Transforms/IndVarSimplify/lftr-reuse.ll
+++ b/test/Transforms/IndVarSimplify/lftr-reuse.ll
@@ -153,6 +153,9 @@ return:
 ; Remove %i which is only used by the exit test.
 ; Verify that SCEV can still compute a backedge count from the sign
 ; extended %n, used for pointer comparison by LFTR.
+;
+; TODO: Fix for PR13371 currently makes this impossible. See
+; IndVarSimplify.cpp hasConcreteDef(). We may want to change to undef rules.
 define void @geplftr(i8* %base, i32 %x, i32 %y, i32 %n) nounwind {
 entry:
   %x.ext = sext i32 %x to i64
@@ -162,13 +165,13 @@ entry:
   %lim = add i32 %x, %n
   %cmp.ph = icmp ult i32 %x, %lim
   br i1 %cmp.ph, label %loop, label %exit
-
+; CHECK: @geplftr
 ; CHECK: loop:
 ; CHECK: phi i8*
-; CHECK-NOT: phi
+; DISABLE-NOT: phi      // This check is currently disabled
 ; CHECK: getelementptr
 ; CHECK: store
-; CHECK: icmp ne i8*
+; DISABLE: icmp ne i8*  // This check is currently disabled
 ; CHECK: br i1
 loop:
   %i = phi i32 [ %x, %entry ], [ %inc, %loop ]
@@ -187,7 +190,7 @@ exit:
 define void @nevertaken() nounwind uwtable ssp {
 entry:
   br label %loop
-
+; CHECK: @nevertaken
 ; CHECK: loop:
 ; CHECK-NOT: phi
 ; CHECK-NOT: add