diff options
| -rw-r--r-- | lib/Target/ARM/ARMInstrInfo.td | 16 |
1 files changed, 6 insertions, 10 deletions
diff --git a/lib/Target/ARM/ARMInstrInfo.td b/lib/Target/ARM/ARMInstrInfo.td index 372277858c..4fa434cba9 100644 --- a/lib/Target/ARM/ARMInstrInfo.td +++ b/lib/Target/ARM/ARMInstrInfo.td @@ -2310,8 +2310,8 @@ def : ARMV6Pat<(or (and GPR:$src1, 0xFFFF0000), defm CMP : AI1_cmp_irs<0b1010, "cmp", BinOpFrag<(ARMcmp node:$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 +// FIXME: We have to be careful when using the CMN instruction and comparison +// with 0. One would expect these two pieces of code should give identical // results: // // rsbs r1, r1, 0 @@ -2336,20 +2336,16 @@ defm CMP : AI1_cmp_irs<0b1010, "cmp", // 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: +// When x is 0 and unsigned: // // 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. +// Therefore, we should disable CMN 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 it's a comparison which doesn't look at the 'carry' flag). // // (See the ARM docs for the "AddWithCarry" pseudo-code.) // |