ARM: Use BKPT instead of TRAP to implement llvm.debugtrap.

[pcc]

Summary

The BKPT instruction is specified to cause a software breakpoint,
and at least on Linux results in a SIGTRAP. This makes it more
suitable for implementing debugtrap than TRAP (aka UDF #254), which
is specified to cause an undefined instruction exception and results
in a SIGILL on Linux.

Moreover, BKPT is not marked as a terminator, which is not only
consistent with the IR instruction but allows the analyzeBlock
function to correctly analyze a basic block containing the instruction,
which fixes an assertion failure in the machine block placement pass
previously triggered by the included test case.

Because BKPT is only supported starting with ARMv5T, we continue to
use UDF #254 when targeting v4T.

[compnerd]

LGTM. WRT the test case, is the DI needed? If not, I would strip the debug info for the test.

[pcc]

Yes, the DI is needed (c-reduce can't remove it, and stripping makes the problem disappear).

llvm/lib/Target/ARM/ARMISelLowering.cpp

for (auto &VT : {MVT::f32, MVT::f64})
       setOperationAction(ISD::FPOWI, VT, Custom);
 
   setOperationAction(ISD::GlobalAddress, MVT::i32,   Custom);
   setOperationAction(ISD::ConstantPool,  MVT::i32,   Custom);
   setOperationAction(ISD::GlobalTLSAddress, MVT::i32, Custom);
   setOperationAction(ISD::BlockAddress, MVT::i32, Custom);
 
   setOperationAction(ISD::TRAP, MVT::Other, Legal);
+  setOperationAction(ISD::DEBUGTRAP, MVT::Other, Legal);
 
   // Use the default implementation.
   setOperationAction(ISD::VASTART,            MVT::Other, Custom);
   setOperationAction(ISD::VAARG,              MVT::Other, Expand);
   setOperationAction(ISD::VACOPY,             MVT::Other, Expand);
   setOperationAction(ISD::VAEND,              MVT::Other, Expand);
   setOperationAction(ISD::STACKSAVE,          MVT::Other, Expand);
   setOperationAction(ISD::STACKRESTORE,       MVT::Other, Expand);

llvm/lib/Target/ARM/ARMInstrInfo.td

 //===----------------------------------------------------------------------===//
 // ARM Instruction Predicate Definitions.
 //
 def HasV4T           : Predicate<"Subtarget->hasV4TOps()">,
                                  AssemblerPredicate<"HasV4TOps", "armv4t">;
 def NoV4T            : Predicate<"!Subtarget->hasV4TOps()">;
 def HasV5T           : Predicate<"Subtarget->hasV5TOps()">,
                                  AssemblerPredicate<"HasV5TOps", "armv5t">;
+def NoV5T            : Predicate<"!Subtarget->hasV5TOps()">;
 def HasV5TE          : Predicate<"Subtarget->hasV5TEOps()">,
                                  AssemblerPredicate<"HasV5TEOps", "armv5te">;
 def HasV6            : Predicate<"Subtarget->hasV6Ops()">,
                                  AssemblerPredicate<"HasV6Ops", "armv6">;
 def NoV6             : Predicate<"!Subtarget->hasV6Ops()">;
 def HasV6M           : Predicate<"Subtarget->hasV6MOps()">,
                                  AssemblerPredicate<"HasV6MOps",
                                                     "armv6m or armv6t2">;

 }
 let isBarrier = 1, isTerminator = 1 in
 def TRAP : AXI<(outs), (ins), MiscFrm, NoItinerary,
                "trap", [(trap)]>,
            Requires<[IsARM,DontUseNaClTrap]> {
   let Inst = 0xe7ffdefe;
 }
 
+def : Pat<(debugtrap), (BKPT 0)>, Requires<[IsARM, HasV5T]>;
+def : Pat<(debugtrap), (UDF 254)>, Requires<[IsARM, NoV5T]>;
+
 // Address computation and loads and stores in PIC mode.
 let isNotDuplicable = 1 in {
 def PICADD  : ARMPseudoInst<(outs GPR:$dst), (ins GPR:$a, pclabel:$cp, pred:$p),
                             4, IIC_iALUr,
                             [(set GPR:$dst, (ARMpic_add GPR:$a, imm:$cp))]>,
                             Sched<[WriteALU, ReadALU]>;
 
 let AddedComplexity = 10 in {

llvm/lib/Target/ARM/ARMInstrThumb.td

 def tUDF : TI<(outs), (ins imm0_255:$imm8), IIC_Br, "udf\t$imm8",
               [(int_arm_undefined imm0_255:$imm8)]>, Encoding16 {
   bits<8> imm8;
   let Inst{15-12} = 0b1101;
   let Inst{11-8} = 0b1110;
   let Inst{7-0} = imm8;
 }
 
+def : Pat<(debugtrap), (tBKPT 0)>, Requires<[IsThumb, HasV5T]>;
+def : Pat<(debugtrap), (tUDF 254)>, Requires<[IsThumb, NoV5T]>;
+
 def t__brkdiv0 : TI<(outs), (ins), IIC_Br, "__brkdiv0",
                     [(int_arm_undefined 249)]>, Encoding16,
     Requires<[IsThumb, IsWindows]> {
   let Inst = 0xdef9;
   let isTerminator = 1;
 }
 
 // Zero-extend byte

llvm/test/CodeGen/ARM/analyze-branch-bkpt.ll

+; RUN: llc -o - %s -mtriple thumbv4-unknown-linux-android | FileCheck --check-prefix=V4 %s
+; RUN: llc -o - %s -mtriple thumbv5-unknown-linux-android | FileCheck --check-prefix=V5 %s
+
+; V4: udf #254
+; V5: bkpt #0
+
+define i1 @a(i32 %b) !dbg !3 {
+  br i1 undef, label %c, label %d, !dbg !4
+
+d:                                                ; preds = %0
+  call void @llvm.debugtrap()
+  br label %ah, !dbg !4
+
+c:                                                ; preds = %0
+  %aj = icmp ne i20 undef, 5
+  br label %ah, !dbg !4
+
+ah:                                               ; preds = %c, %d
+  %ak = phi i1 [ false, %d ], [ %aj, %c ]
+  call void @llvm.dbg.value(metadata i1 %ak, metadata !7, metadata !DIExpression()), !dbg !9
+  switch i32 %b, label %al [
+    i32 0, label %am
+    i32 10, label %an
+  ]
+
+an:                                               ; preds = %ah
+  %ch = select i1 %ak, i32 0, i32 5
+  br label %am, !dbg !10
+
+al:                                               ; preds = %ah
+  br label %am, !dbg !9
+
+am:                                               ; preds = %al, %an, %ah
+  %1 = phi i32 [ 0, %al ], [ %ch, %an ], [ %b, %ah ]
+  unreachable
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+; Function Attrs: nounwind
+declare void @llvm.debugtrap() #1
+
+attributes #0 = { nounwind readnone speculatable }
+attributes #1 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, isOptimized: false, runtimeVersion: 0, emissionKind: NoDebug)
+!1 = !DIFile(filename: "a", directory: "")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = distinct !DISubprogram(scope: null, isLocal: false, isDefinition: true, isOptimized: false, unit: !0)
+!4 = !DILocation(line: 0, scope: !5, inlinedAt: !6)
+!5 = distinct !DISubprogram(scope: null, isLocal: false, isDefinition: true, isOptimized: false, unit: !0)
+!6 = !DILocation(line: 0, scope: !3)
+!7 = !DILocalVariable(scope: !8)
+!8 = distinct !DISubprogram(scope: null, isLocal: false, isDefinition: true, isOptimized: false, unit: !0)
+!9 = !DILocation(line: 0, scope: !8, inlinedAt: !6)
+!10 = !DILocation(line: 0, scope: !11, inlinedAt: !6)
+!11 = !DILexicalBlock(scope: !8)

llvm/test/CodeGen/ARM/debugtrap.ll

 ; This test ensures the @llvm.debugtrap() call is not removed when generating
 ; the 'pop' instruction to restore the callee saved registers on ARM.
 
-; RUN: llc < %s -mtriple=armv7 -O0 -filetype=asm | FileCheck %s 
+; RUN: llc < %s -mtriple=armv4 -O0 -filetype=asm | FileCheck --check-prefixes=CHECK,V4 %s
+; RUN: llc < %s -mtriple=armv5 -O0 -filetype=asm | FileCheck --check-prefixes=CHECK,V5 %s
+; RUN: llc < %s -mtriple=thumbv4 -O0 -filetype=asm | FileCheck --check-prefixes=CHECK,V4 %s
+; RUN: llc < %s -mtriple=thumbv5 -O0 -filetype=asm | FileCheck --check-prefixes=CHECK,V5 %s
 
 declare void @llvm.debugtrap() nounwind
 declare void @foo() nounwind
 
 define void @test() nounwind {
 entry:
   ; CHECK: bl foo
+  ; V4-NEXT: udf #254
+  ; V5-NEXT: bkpt #0
   ; CHECK-NEXT: pop
-  ; CHECK-NEXT: .inst 0xe7ffdefe
   call void @foo()
   call void @llvm.debugtrap()
   ret void
 }

llvm/test/CodeGen/ARM/trap.ll

; ENCODING-THUMB: fe de trap
 
   call void @llvm.trap()
   unreachable
 }
 
 define void @t2() nounwind {
 entry:
 ; DARWIN-LABEL: t2:
-; DARWIN: trap
+; DARWIN: udf #254
 
 ; FUNC-LABEL: t2:
 ; FUNC: bl __trap
 
 ; NACL-LABEL: t2:
-; NACL: .inst 0xe7fedef0
+; NACL: bkpt #0
 
 ; ARM-LABEL: t2:
-; ARM: .inst 0xe7ffdefe
+; ARM: bkpt #0
 
 ; THUMB-LABEL: t2:
-; THUMB: .inst.n 0xdefe
+; THUMB: bkpt #0
 
-; ENCODING-NACL: f0 de fe e7 trap
+; ENCODING-NACL: 70 00 20 e1 bkpt #0
 
-; ENCODING-ARM: fe de ff e7 trap
+; ENCODING-ARM: 70 00 20 e1 bkpt #0
 
-; ENCODING-THUMB: fe de trap
+; ENCODING-THUMB: 00 be bkpt #0
 
   call void @llvm.debugtrap()
   unreachable
 }
 
 declare void @llvm.trap() nounwind
 declare void @llvm.debugtrap() nounwind