Revert r112176; it broke test/CodeGen/Thumb2/thumb2-cmn.ll.

llvm-svn: 112191

1 files changed, 8 insertions, 50 deletions

diff --git a/llvm/lib/Target/ARM/ARMInstrThumb2.td b/llvm/lib/Target/ARM/ARMInstrThumb2.td
index d013922ec94..7372631fe5f 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb2.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb2.td
@@ -2141,60 +2141,18 @@ defm t2CMP  : T2I_cmp_irs<0b1101, "cmp",
 defm t2CMPz : T2I_cmp_irs<0b1101, "cmp",
                           BinOpFrag<(ARMcmpZ node:$LHS, node:$RHS)>>;
 
-// FIXME: There seems to be a (potential) hardware bug with the CMN instruction
-// and comparison with 0. These two pieces of code should give identical
-// results:
-//
-//   rsbs r1, r1, 0
-//   cmp  r0, r1
-//   mov  r0, #0
-//   it   ls
-//   mov  r0, #1
-//
-// and:
-// 
-//   cmn  r0, r1
-//   mov  r0, #0
-//   it   ls
-//   mov  r0, #1
-//
-// However, the CMN gives the *opposite* result when r1 is 0. This is because
-// the carry flag is set in the CMP case but not in the CMN case. In short, the
-// CMP instruction doesn't perform a truncate of the (logical) NOT of 0 plus the
-// value of r0 and the carry bit (because the "carry bit" parameter to
-// AddWithCarry is defined as 1 in this case, the carry flag will always be set
-// when r0 >= 0). The CMN instruction doesn't perform a NOT of 0 so there is
-// never a "carry" when this AddWithCarry is performed (because the "carry bit"
-// parameter to AddWithCarry is defined as 0).
-//
-// The AddWithCarry in the CMP case seems to be relying upon the identity:
-// 
-//   ~x + 1 = -x
-//
-// However when x is 0 and unsigned, this doesn't hold:
-//
-//    x = 0
-//   ~x = 0xFFFF FFFF
-//   ~x + 1 = 0x1 0000 0000
-//   (-x = 0) != (0x1 0000 0000 = ~x + 1)
-//
-// Therefore, we should disable *all* versions of CMN, especially when comparing
-// against zero, until we can limit when the CMN instruction is used (when we
-// know that the RHS is not 0) or when we have a hardware fix for this.
-//
-// (See the ARM docs for the "AddWithCarry" pseudo-code.)
-//
-// This is related to <rdar://problem/7569620>.
-//
+//FIXME: Disable CMN, as CCodes are backwards from compare expectations
+//       Compare-to-zero still works out, just not the relationals
 //defm t2CMN  : T2I_cmp_irs<0b1000, "cmn",
 //                          BinOpFrag<(ARMcmp node:$LHS,(ineg node:$RHS))>>;
-//defm t2CMNz : T2I_cmp_irs<0b1000, "cmn",
-//                          BinOpFrag<(ARMcmpZ node:$LHS,(ineg node:$RHS))>>;
-//
+defm t2CMNz : T2I_cmp_irs<0b1000, "cmn",
+                          BinOpFrag<(ARMcmpZ node:$LHS,(ineg node:$RHS))>>;
+
 //def : T2Pat<(ARMcmp  GPR:$src, t2_so_imm_neg:$imm),
 //            (t2CMNri GPR:$src, t2_so_imm_neg:$imm)>;
-//def : T2Pat<(ARMcmpZ  GPR:$src, t2_so_imm_neg:$imm),
-//            (t2CMNzri GPR:$src, t2_so_imm_neg:$imm)>;
+
+def : T2Pat<(ARMcmpZ  GPR:$src, t2_so_imm_neg:$imm),
+            (t2CMNzri GPR:$src, t2_so_imm_neg:$imm)>;
 
 defm t2TST  : T2I_cmp_irs<0b0000, "tst",
                           BinOpFrag<(ARMcmpZ (and node:$LHS, node:$RHS), 0)>>;