ARM: Don't form a t2LDRi8 or t2STRi8 with an offset of zero.

The load/store optimizer splits LDRD/STRD into two instructions when the
register pairing doesn't work out. For negative offsets in Thumb2, it uses
t2STRi8 to do that. That's fine, except for the case when the offset is in
the range [-4,-1]. In that case, we'll also form a second t2STRi8 with
the original offset plus 4, resulting in a t2STRi8 with a non-negative
offset, which ends up as if it were an STRT, which is completely bogus.
Similarly for loads.

No testcase, unfortunately, as any I've been able to construct is both large
and extremely fragile.

rdar://11193937

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154141 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/ARM/ARMLoadStoreOptimizer.cpp b/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
index 5770f7739ae6d7d731e4bd7d9c61a58aac8acdc0..b632c78700b9bd5b3e4c05a4526373864c3bca83 100644 (file)
--- a/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
+++ b/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
@@ -1172,6 +1172,10 @@ bool ARMLoadStoreOpt::FixInvalidRegPairOp(MachineBasicBlock &MBB,
                       BaseReg, false, BaseUndef, false, OffUndef,
                       Pred, PredReg, TII, isT2);
         NewBBI = llvm::prior(MBBI);
+        // Be extra careful for thumb2. t2LDRi8 can't reference a zero offset,
+        // so adjust and use t2LDRi12 here for that.
+        if (isT2 && NewOpc == ARM::t2LDRi8 && OffImm+4 >= 0)
+          NewOpc = ARM::t2LDRi12;
         InsertLDR_STR(MBB, MBBI, OffImm, isLd, dl, NewOpc,
                       EvenReg, EvenDeadKill, false,
                       BaseReg, BaseKill, BaseUndef, OffKill, OffUndef,
@@ -1193,6 +1197,10 @@ bool ARMLoadStoreOpt::FixInvalidRegPairOp(MachineBasicBlock &MBB,
                       BaseReg, false, BaseUndef, false, OffUndef,
                       Pred, PredReg, TII, isT2);
         NewBBI = llvm::prior(MBBI);
+        // Be extra careful for thumb2. t2STRi8 can't reference a zero offset,
+        // so adjust and use t2STRi12 here for that.
+        if (isT2 && NewOpc == ARM::t2STRi8 && OffImm+4 >= 0)
+          NewOpc = ARM::t2STRi12;
         InsertLDR_STR(MBB, MBBI, OffImm+4, isLd, dl, NewOpc,
                       OddReg, OddDeadKill, OddUndef,
                       BaseReg, BaseKill, BaseUndef, OffKill, OffUndef,