[ARM] MSR instructions implicitly set CPSR

[olista01]

Summary

The MSR instructions can write to the CPSR, but we did not model this
fact, so we could emit them in the middle of IT blocks, changing the
condition flags for later instructions in the block.

The tests use two calls to llvm.write_register.i32 because it is valid
to use these instructions at the end of an IT block, which if conversion
does do in some cases. With two calls, the first clobbers the flags, so
a branch has to be used to make the second one conditional.

[rengolin]

Ouch! LGTM, thanks!

lib/Target/ARM/ARMInstrInfo.td

 
 // Move from ARM core register to Special Register
 //
 // No need to have both system and application versions of MSR (immediate) or
 // MSR (register), the encodings are the same and the assembly parser has no way
 // to distinguish between them. The mask operand contains the special register
 // (R Bit) in bit 4 and bits 3-0 contains the mask with the fields to be
 // accessed in the special register.
+let Defs = [CPSR] in
 def MSR : ABI<0b0001, (outs), (ins msr_mask:$mask, GPR:$Rn), NoItinerary,
               "msr", "\t$mask, $Rn", []> {
   bits<5> mask;
   bits<4> Rn;
 
   let Inst{23} = 0;
   let Inst{22} = mask{4}; // R bit
   let Inst{21-20} = 0b10;
   let Inst{19-16} = mask{3-0};
   let Inst{15-12} = 0b1111;
   let Inst{11-4} = 0b00000000;
   let Inst{3-0} = Rn;
 }
 
+let Defs = [CPSR] in
 def MSRi : ABI<0b0011, (outs), (ins msr_mask:$mask,  mod_imm:$imm), NoItinerary,
                "msr", "\t$mask, $imm", []> {
   bits<5> mask;
   bits<12> imm;
 
   let Inst{23} = 0;
   let Inst{22} = mask{4}; // R bit
   let Inst{21-20} = 0b10;

lib/Target/ARM/ARMInstrThumb2.td

 // Move from ARM core register to Special Register
 //
 // A/R class MSR.
 //
 // No need to have both system and application versions, the encodings are the
 // same and the assembly parser has no way to distinguish between them. The mask
 // operand contains the special register (R Bit) in bit 4 and bits 3-0 contains
 // the mask with the fields to be accessed in the special register.
+let Defs = [CPSR] in
 def t2MSR_AR : T2I<(outs), (ins msr_mask:$mask, rGPR:$Rn),
                    NoItinerary, "msr", "\t$mask, $Rn", []>,
                Requires<[IsThumb2,IsNotMClass]> {
   bits<5> mask;
   bits<4> Rn;
   let Inst{31-21} = 0b11110011100;
   let Inst{20}    = mask{4}; // R Bit
   let Inst{19-16} = Rn;
def t2MSRbanked : T2I<(outs), (ins banked_reg:$banked, rGPR:$Rn),
   let Inst{4} = banked{4};
   let Inst{3-0} = 0b0000;
 }
 
 
 // M class MSR.
 //
 // Move from ARM core register to Special Register
+let Defs = [CPSR] in
 def t2MSR_M : T2I<(outs), (ins msr_mask:$SYSm, rGPR:$Rn),
                   NoItinerary, "msr", "\t$SYSm, $Rn", []>,
               Requires<[IsThumb,IsMClass]> {
   bits<12> SYSm;
   bits<4> Rn;
   let Inst{31-21} = 0b11110011100;
   let Inst{20}    = 0b0;
   let Inst{19-16} = Rn;

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

+; RUN: llc < %s -mtriple=thumbv6m-none-eabi | FileCheck %s --check-prefix=V6M --check-prefix=CHECK
+; RUN: llc < %s -mtriple=thumbv7m-none-eabi | FileCheck %s --check-prefix=V7M --check-prefix=CHECK
+; RUN: llc < %s -mtriple=thumbv7a-none-eabi | FileCheck %s --check-prefix=V7A --check-prefix=CHECK
+; RUN: llc < %s -mtriple=armv7a-none-eabi   | FileCheck %s --check-prefix=V7A --check-prefix=CHECK
+
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7a-arm-none-eabi"
+
+define void @test_const(i32 %val) {
+; CHECK-LABEL: test_const:
+entry:
+  %cmp = icmp eq i32 %val, 0
+  br i1 %cmp, label %write_reg, label %exit
+
+write_reg:
+  tail call void @llvm.write_register.i32(metadata !0, i32 0)
+  tail call void @llvm.write_register.i32(metadata !0, i32 0)
+; V6M: msr     apsr, {{r[0-9]+}}
+; V6M: msr     apsr, {{r[0-9]+}}
+; V7M: msr     apsr_nzcvq, {{r[0-9]+}}
+; V7M: msr     apsr_nzcvq, {{r[0-9]+}}
+; V7A: msr     APSR_nzcvqg, {{r[0-9]+}}
+; V7A: msr     APSR_nzcvqg, {{r[0-9]+}}
+  br label %exit
+
+exit:
+  ret void
+}
+
+define void @test_var(i32 %val, i32 %apsr) {
+; CHECK-LABEL: test_var:
+entry:
+  %cmp = icmp eq i32 %val, 0
+  br i1 %cmp, label %write_reg, label %exit
+
+write_reg:
+  tail call void @llvm.write_register.i32(metadata !0, i32 %apsr)
+  tail call void @llvm.write_register.i32(metadata !0, i32 %apsr)
+; V6M: msr     apsr, {{r[0-9]+}}
+; V6M: msr     apsr, {{r[0-9]+}}
+; V7M: msr     apsr_nzcvq, {{r[0-9]+}}
+; V7M: msr     apsr_nzcvq, {{r[0-9]+}}
+; V7A: msr     APSR_nzcvqg, {{r[0-9]+}}
+; V7A: msr     APSR_nzcvqg, {{r[0-9]+}}
+  br label %exit
+
+exit:
+  ret void
+}
+
+
+declare void @llvm.write_register.i32(metadata, i32)
+
+!0 = !{!"apsr"}