[PATCH] D51165: [CodeGen] emit inline asm clobber list warnings for reserved (cont)

[Ties Stuij via Phabricator via llvm-commits]

stuij created this revision.
stuij added reviewers: efriedma, olista01, javed.absar.
Herald added subscribers: llvm-commits, kristof.beyls, eraman.

This is a continuation of https://reviews.llvm.org/D49727
Below the original text, current changes in the comments:

Currently, in line with GCC, when specifying reserved registers like sp or pc on an inline asm() clobber list, we don't always preserve the original value across the statement. And in general, overwriting reserved registers can have surprising results.

For example:

  extern int bar(int[]);
  
  int foo(int i) {
    int a[i]; // VLA
    asm volatile(
        "mov r7, #1"
      :
      :
      : "r7"
    );
  
    return 1 + bar(a);
  }

Compiled for thumb, this gives:

  $ clang --target=arm-arm-none-eabi -march=armv7a -c test.c -o - -S -O1 -mthumb
  ...
  foo:
          .fnstart
  @ %bb.0:                                @ %entry
          .save   {r4, r5, r6, r7, lr}
          push    {r4, r5, r6, r7, lr}
          .setfp  r7, sp, #12
          add     r7, sp, #12
          .pad    #4
          sub     sp, #4
          movs    r1, #7
          add.w   r0, r1, r0, lsl #2
          bic     r0, r0, #7
          sub.w   r0, sp, r0
          mov     sp, r0
          @APP
          mov.w   r7, #1
          @NO_APP
          bl      bar
          adds    r0, #1
          sub.w   r4, r7, #12
          mov     sp, r4
          pop     {r4, r5, r6, r7, pc}
  ...

r7 is used as the frame pointer for thumb targets, and this function needs to restore the SP from the FP because of the variable-length stack allocation a. r7 is clobbered by the inline assembly (and r7 is included in the clobber list), but LLVM does not preserve the value of the frame pointer across the assembly block.

This type of behavior is similar to GCC's and has been discussed on the bugtracker: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11807 . No consensus seemed to have been reached on the way forward. Clang behavior has briefly been discussed on the CFE mailing (starting here: http://lists.llvm.org/pipermail/cfe-dev/2018-July/058392.html). I've opted for following Eli Friedman's advice to print warnings when there are reserved registers on the clobber list so as not to diverge from GCC behavior for now.

The patch uses MachineRegisterInfo's target-specific knowledge of reserved registers, just before we convert the inline asm string in the AsmPrinter.

If we find a reserved register, we print a warning:

  repro.c:6:7: warning: inline asm clobber list contains reserved registers: R7 [-Winline-asm]
        "mov r7, #1"
        ^


Repository:
  rL LLVM

https://reviews.llvm.org/D51165

Files:
  include/llvm/CodeGen/AsmPrinter.h
  include/llvm/CodeGen/TargetRegisterInfo.h
  lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp
  lib/Target/AArch64/AArch64RegisterInfo.cpp
  lib/Target/AArch64/AArch64RegisterInfo.h
  lib/Target/ARM/ARMBaseRegisterInfo.cpp
  lib/Target/ARM/ARMBaseRegisterInfo.h
  test/CodeGen/AArch64/inline-asm-clobber.ll
  test/CodeGen/ARM/inline-asm-clobber.ll

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