Cmp peephole optimization isn't always safe for signed arithmetics.

int tries = INT_MAX;    
while (tries > 0) {
      tries--;
}

The check should be:
        subs    r4, #1
        cmp     r4, #0
        bgt     LBB0_1

The subs can set the overflow V bit when r4 is INT_MAX+1 (which loop
canonicalization apparently does in this case). cmp #0 would have cleared
it while not changing the N and Z bits. Since BGT is dependent on the V
bit, i.e. (N == V) && !Z, it is not safe to eliminate the cmp #0.

rdar://9172742


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128179 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 41 insertions, 0 deletions

diff --git a/test/CodeGen/ARM/2011-03-23-PeepholeBug.ll b/test/CodeGen/ARM/2011-03-23-PeepholeBug.ll
new file mode 100644
index 0000000000..7c9af6f5e5
--- /dev/null
+++ b/test/CodeGen/ARM/2011-03-23-PeepholeBug.ll
@@ -0,0 +1,41 @@
+; RUN: llc < %s -mtriple=thumbv7-apple-darwin10 -relocation-model=pic -disable-fp-elim -mcpu=cortex-a8 | FileCheck %s
+
+; subs r4, #1
+; cmp r4, 0
+; bgt
+; cmp cannot be optimized away since it will clear the overflow bit.
+; gt / ge, lt, le conditions all depend on V bit.
+; rdar://9172742
+
+define i32 @t() nounwind {
+; CHECK: t:
+entry:
+  br label %bb2
+
+bb:                                               ; preds = %bb2
+  %0 = tail call i32 @rand() nounwind
+  %1 = icmp eq i32 %0, 50
+  br i1 %1, label %bb3, label %bb1
+
+bb1:                                              ; preds = %bb
+  %tmp = tail call i32 @puts() nounwind
+  %indvar.next = add i32 %indvar, 1
+  br label %bb2
+
+bb2:                                              ; preds = %bb1, %entry
+; CHECK: bb2
+; CHECK: subs [[REG:r[0-9]+]], #1
+; CHECK: cmp [[REG]], #0
+; CHECK: bgt
+  %indvar = phi i32 [ %indvar.next, %bb1 ], [ 0, %entry ]
+  %tries.0 = sub i32 2147483647, %indvar
+  %tmp1 = icmp sgt i32 %tries.0, 0
+  br i1 %tmp1, label %bb, label %bb3
+
+bb3:                                              ; preds = %bb2, %bb
+  ret i32 0
+}
+
+declare i32 @rand()
+
+declare i32 @puts() nounwind