ARM: make Thumb1 instructions non-flag-setting in IT block.

Many Thumb1 instructions are defined to set CPSR if executed outside an IT
block, but leave it alone from inside one. In MachineIR this is represented by
whether an optional register is CPSR or NoReg (0), and affects how the
instructions are printed.

This sets the instruction to the appropriate form during if-conversion.

lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -537,6 +537,18 @@ bool ARMBaseInstrInfo::PredicateInstruction(
     MachineOperand &PMO = MI.getOperand(PIdx);
     PMO.setImm(Pred[0].getImm());
     MI.getOperand(PIdx+1).setReg(Pred[1].getReg());
+
+    // Thumb 1 arithmetic instructions do not set CPSR when executed inside an
+    // IT block. This affects how they are printed.
+    const MCInstrDesc &MCID = MI.getDesc();
+    if (MCID.TSFlags & ARMII::ThumbArithFlagSetting) {
+      assert(MCID.OpInfo[1].isOptionalDef() && "CPSR def isn't expected operand");
+      assert((MI.getOperand(1).isDead() ||
+              MI.getOperand(1).getReg() != ARM::CPSR) &&
+             "if conversion tried to stop defining used CPSR");
+      MI.getOperand(1).setReg(ARM::NoRegister);
+    }
+
     return true;
   }
   return false;

lib/Target/ARM/ARMInstrFormats.td

@@ -403,8 +403,9 @@ class InstTemplate<AddrMode am, int sz, IndexMode im,
   bit isUnaryDataProc = 0;
   bit canXformTo16Bit = 0;
   // The instruction is a 16-bit flag setting Thumb instruction. Used
-  // by the parser to determine whether to require the 'S' suffix on the
-  // mnemonic (when not in an IT block) or preclude it (when in an IT block).
+  // by the parser and if-converter to determine whether to require the 'S'
+  // suffix on the mnemonic (when not in an IT block) or preclude it (when
+  // in an IT block).
   bit thumbArithFlagSetting = 0;
 
   bit validForTailPredication = 0;

test/CodeGen/ARM/thumb2-it-block.ll

@@ -16,9 +16,7 @@ entry:
 
 ; CHECK:        cmp
 ; CHECK-NEXT:   it    mi
-; We shouldn't need to check for the extra 's' here; tRSB should be printed as
-; "rsb" inside an IT block, not "rsbs".
-; CHECK-NEXT:   rsb{{s?}}mi
+; CHECK-NEXT:   rsbmi
 ; CHECK-NEXT:   cmp
 ; CHECK-NEXT:   it    mi
 ; CHECK-NEXT:   rsb{{s?}}mi

test/CodeGen/Thumb2/ifcvt-rescan-diamonds.ll

@@ -22,7 +22,8 @@ while.cond38:                                     ; preds = %if.end111, %entry
 ; CHECK-NEXT: it eq
 ; CHECK-NEXT: ldreq
 ; CHECK-NEXT: it ne
-; CHECK-NEXT: movsne
+  ; N.b. 16-bit mov instruction in IT block does not set flags.
+; CHECK-NEXT: movne
 ; CHECK-NEXT: mvns
 ; CHECK-NEXT: b
 cond.true77:                                      ; preds = %while.cond38