diff options
| -rw-r--r-- | lib/Target/ARM/ARMInstrThumb2.td | 58 |
1 files changed, 50 insertions, 8 deletions
diff --git a/lib/Target/ARM/ARMInstrThumb2.td b/lib/Target/ARM/ARMInstrThumb2.td index 7372631..d013922 100644 --- a/lib/Target/ARM/ARMInstrThumb2.td +++ b/lib/Target/ARM/ARMInstrThumb2.td @@ -2141,18 +2141,60 @@ defm t2CMP : T2I_cmp_irs<0b1101, "cmp", defm t2CMPz : T2I_cmp_irs<0b1101, "cmp", BinOpFrag<(ARMcmpZ node:$LHS, node:$RHS)>>; -//FIXME: Disable CMN, as CCodes are backwards from compare expectations -// Compare-to-zero still works out, just not the relationals +// 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>. +// //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)>>; |