[AArch64] Add NZCV Def for TLSDESC_CALLSEQ

[MuellerMP]

Summary

The glibc and older musl handlers of tlsdesc_dynamic use a cmp instruction which will clobber NZCV.

See glibc's _dl_tlsdesc_dynamic:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aarch64/dl-tlsdesc.S;hb=refs/heads/release/2.37/master

See v1.1.21 Musl's __tlsdesc_dynamic:
https://git.musl-libc.org/cgit/musl/tree/src/ldso/aarch64/tlsdesc.s?h=v1.1.21

[dmgreen]

This sounds good, as far as I understand. Can you run update_llc_test_checks on the test file to generate the output, perhaps with a comment explaining what it is testing?

[MuellerMP]

Ran update_llc_test_checks on the test and added a comment to it.

[MuellerMP]

In my research for this case i stumbled over the glibc arm implementation and a TLSDESC RFC targeted towards arm.
They provide information as to why flags is assumed to be clobbered in this case.

References:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/arm/dl-tlsdesc.S;hb=refs/heads/release/2.37/master#l85
https://www.fsfla.org/~lxoliva/writeups/TLS/RFC-TLSDESC-ARM.txt

I'm not too sure though if a "-stop-after=irtranslator" test for the "implicit-def dead $nzcv" might be a better unit test.
Right now my test seems much more like a integration type of test which asserts the implications of the def.

[dmgreen]

Thanks. LGTM

[MuellerMP]

Hey!
Since I don't have commit access, I would appreciate if someone else could commit this.

[dmgreen]

Oh yeah - can do! What "name <[e-mail]>" should be used for attribution?

[MuellerMP]

Thanks! You can use: "MuellerMP <[e-mail]>"

llvm/lib/Target/AArch64/AArch64InstrInfo.td

 
 // Pseudo instruction to tell the streamer to emit a 'G' character into the
 // augmentation string.
 def EMITMTETAGGED : Pseudo<(outs), (ins), []>, Sched<[]> {}
 
 // FIXME: maybe the scratch register used shouldn't be fixed to X1?
 // FIXME: can "hasSideEffects be dropped?
 // This gets lowered to an instruction sequence which takes 16 bytes
-let isCall = 1, Defs = [LR, X0, X1], hasSideEffects = 1, Size = 16,
+let isCall = 1, Defs = [NZCV, LR, X0, X1], hasSideEffects = 1, Size = 16,
     isCodeGenOnly = 1 in
 def TLSDESC_CALLSEQ
     : Pseudo<(outs), (ins i64imm:$sym),
              [(AArch64tlsdesc_callseq tglobaltlsaddr:$sym)]>,
       Sched<[WriteI, WriteLD, WriteI, WriteBrReg]>;
 def : Pat<(AArch64tlsdesc_callseq texternalsym:$sym),
           (TLSDESC_CALLSEQ texternalsym:$sym)>;
 

llvm/test/CodeGen/AArch64/aarch64-tls-flags.ll

+; RUN: llc -mtriple=aarch64-unknown-linux-gnu -relocation-model=pic -verify-machineinstrs %s -o - | FileCheck %s
+
+; TLSDESC resolver calling convention does not retain the flags register.
+; Check that a TLS descriptor call cannot be lowered in between a cmp and the use of flags.
+
+@var = thread_local global i32 zeroinitializer
+@test = global i32 zeroinitializer
+
+define i32 @test_thread_local() {
+; CHECK-LABEL: test_thread_local:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    str x30, [sp, #-16]! // 8-byte Folded Spill
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    .cfi_offset w30, -16
+; CHECK-NEXT:    adrp x8, :got:test
+; CHECK-NEXT:    ldr x8, [x8, :got_lo12:test]
+; CHECK-NEXT:    adrp x0, :tlsdesc:var
+; CHECK-NEXT:    ldr x1, [x0, :tlsdesc_lo12:var]
+; CHECK-NEXT:    add x0, x0, :tlsdesc_lo12:var
+; CHECK-NEXT:    .tlsdesccall var
+; CHECK-NEXT:    blr x1
+; CHECK-NEXT:    mrs x9, TPIDR_EL0
+; CHECK-NEXT:    ldr w9, [x9, x0]
+; CHECK-NEXT:    cmp x8, #0
+; CHECK-NEXT:    cinc w0, w9, eq
+; CHECK-NEXT:    ldr x30, [sp], #16 // 8-byte Folded Reload
+; CHECK-NEXT:    ret
+
+  %testval = load i32, ptr @test
+  %test = icmp eq ptr @test, null
+  %val = load i32, ptr @var
+  %result = zext i1 %test to i32
+  %result2 = add i32 %val, %result
+  ret i32 %result2
+
+}