[ARM] Be less conservative about forming IT blocks.

[jmolloy]

Summary

In http://reviews.llvm.org/rL215382, IT forming was made more conservative under
the belief that a flag-setting instruction was unpredictable inside an IT block on ARMv6M.

But actually, ARMv6M doesn't even support IT blocks so that's impossible. In the ARMARM for
v7M, v7AR and v8AR it states that the semantics of such an instruction changes inside an
IT block - it doesn't set the flags. So actually it is fine to use one inside an IT block
as long as the flags register is dead afterwards.

[rengolin]

LGTM. Thanks!

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

20 ↗

(On Diff #31124)

Have you opened a bug for this?

llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp

for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     }
   }
 
   return Found;
 }
 
 static bool isCPSRDefined(const MachineInstr *MI) {
   for (const auto &MO : MI->operands())
-    if (MO.isReg() && MO.getReg() == ARM::CPSR && MO.isDef())
+    if (MO.isReg() && MO.getReg() == ARM::CPSR && MO.isDef() && !MO.isDead())
       return true;
   return false;
 }
 
 static bool isEligibleForITBlock(const MachineInstr *MI) {
   switch (MI->getOpcode()) {
   default: return true;
   case ARM::tADC:   // ADC (register) T1

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

-; RUN: llc -mtriple=thumb-eabi -mcpu=arm1156t2-s -mattr=+thumb2 %s -o - | FileCheck -check-prefix CHECK-V7 %s
-; RUN: llc -mtriple=thumbv8 %s -o - | FileCheck %s -check-prefix CHECK-V8
+; RUN: llc -mtriple=thumb-eabi -mcpu=arm1156t2-s -mattr=+thumb2 %s -o - | FileCheck %s
+; RUN: llc -mtriple=thumbv8 %s -o - | FileCheck %s
 ; PR11107
 
 define i32 @test(i32 %a, i32 %b) {
 entry:
  %cmp1 = icmp slt i32 %a, 0
  %sub1 = sub nsw i32 0, %a
  %abs1 = select i1 %cmp1, i32 %sub1, i32 %a
  %cmp2 = icmp slt i32 %b, 0
  %sub2 = sub nsw i32 0, %b
  %abs2 = select i1 %cmp2, i32 %sub2, i32 %b
  %add = add nsw i32 %abs1, %abs2
  ret i32 %add
 }
 
-; CHECK-V7:        cmp
-; CHECK-V7-NEXT:   it    mi
-; CHECK-V7-NEXT:   rsbmi
-; CHECK-V7-NEXT:   cmp
-; CHECK-V7-NEXT:   it    mi
-; CHECK-V7-NEXT:   rsbmi
+; 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:   cmp
+; CHECK-NEXT:   it    mi
+; CHECK-NEXT:   rsb{{s?}}mi
 
-; CHECK-V8:        cmp
-; CHECK-V8-NEXT:   bpl
-; CHECK-V8:        rsbs
-; CHECK-V8:        cmp
-; CHECK-V8-NEXT:   bpl
-; CHECK-V8:        rsbs