Fix handling of negative offsets for AddrModeT2_i8s4 in rewriteT2FrameIndex.

This is a follow up to r230233 to fix something that I noticed by
inspection. The AddrModeT2_i8s4 addressing mode does not support
negative offsets. I spent a good chunk of the day trying to come up with
a testcase for this but was not successful. This addressing mode is used
to spill and restore GPRPair registers in Thumb2 code and that does not
happen often. We also make very limited used of negative offsets when
lowering frame indexes. I am going ahead with the change anyway, because
I am pretty confident that it is correct. I also added a missing assertion
to check that the low bits of the scaled offset are zero.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230297 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/ARM/Thumb2InstrInfo.cpp b/lib/Target/ARM/Thumb2InstrInfo.cpp
index 2085bfa9f166fc492e26ea75a499edbab6a780b2..62c37526381c175dbd4f71ac1745cc4724e72a00 100644 (file)
--- a/lib/Target/ARM/Thumb2InstrInfo.cpp
+++ b/lib/Target/ARM/Thumb2InstrInfo.cpp
@@ -575,12 +575,9 @@ bool llvm::rewriteT2FrameIndex(MachineInstr &MI, unsigned FrameRegIdx,
     } else if (AddrMode == ARMII::AddrModeT2_i8s4) {
       Offset += MI.getOperand(FrameRegIdx + 1).getImm() * 4;
       NumBits = 10; // 8 bits scaled by 4
-      // MCInst operand has already scaled value.
+      // MCInst operand expects already scaled value.
       Scale = 1;
-      if (Offset < 0) {
-        isSub = true;
-        Offset = -Offset;
-      }
+      assert((Offset & 3) == 0 && "Can't encode this offset!");
     } else {
       llvm_unreachable("Unsupported addressing mode!");
     }