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| author | Dan Gohman <gohman@apple.com> | 2010-08-26 15:50:25 +0000 |
|---|---|---|
| committer | Dan Gohman <gohman@apple.com> | 2010-08-26 15:50:25 +0000 |
| commit | 4b7dff9a793f3258936c990c8f6a991366d4f55b (patch) | |
| tree | bcf9593e830741db2fefa8cd89707355c263d0fa /lib/Target/ARM/ARMInstrThumb2.td | |
| parent | 6cb8c23db1c3becdce6dfbf1b7f1677faca4251e (diff) | |
| download | external_llvm-4b7dff9a793f3258936c990c8f6a991366d4f55b.zip external_llvm-4b7dff9a793f3258936c990c8f6a991366d4f55b.tar.gz external_llvm-4b7dff9a793f3258936c990c8f6a991366d4f55b.tar.bz2 | |
Revert r112176; it broke test/CodeGen/Thumb2/thumb2-cmn.ll.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112191 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Target/ARM/ARMInstrThumb2.td')
| -rw-r--r-- | lib/Target/ARM/ARMInstrThumb2.td | 58 |
1 files changed, 8 insertions, 50 deletions
diff --git a/lib/Target/ARM/ARMInstrThumb2.td b/lib/Target/ARM/ARMInstrThumb2.td index d013922..7372631 100644 --- a/lib/Target/ARM/ARMInstrThumb2.td +++ b/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)>>; |