[PATCH] D156497: [BPF] Emit UNDEF rather than constant 0

[Eduard Zingerman via Phabricator via llvm-commits]

eddyz87 added a comment.

By the way, it is still possible to test this using e.g. llc or clang. As noted previously in this thread the default error handler for llc/clang is stateful, so it proceeds the code generation, produces assembly code and returns non-zero error code.

I checked how `UNDEF` is handled, short answer: it disappears and it could replace instructions that use it.
For example:

  $ cat t.c
  extern void bar(void);
  int foo(int a, int b, int c, int d, int e, int f) {
    bar();
    return a + f;
  }
  $ clang -O2 --target=bpf -mllvm -print-after=verify,bpf-isel,processimpdefs -c t.c -o - | llvm-objdump -d -
  *** IR Dump After Module Verifier (verify) ***
  ; Function Attrs: nounwind
  define dso_local i32 @foo(i32 noundef %a, i32 noundef %b, i32 noundef %c, i32 noundef %d, i32 noundef %e, i32 noundef %f) local_unnamed_addr #0 {
  entry:
    %call = tail call i32 @bar() #2
    %add = add nsw i32 %f, %a
    ret i32 %add
  }
  t.c:2:5: error: stack arguments are not supported
      2 | int foo(int a, int b, int c, int d, int e, int f) {
        |     ^
  # *** IR Dump After BPF DAG->DAG Pattern Instruction Selection (bpf-isel) ***:
  # Machine code for function foo: IsSSA, TracksLiveness
  
  bb.0.entry:
    ADJCALLSTACKDOWN 0, 0, implicit-def dead $r11, implicit $r11
    JAL @bar, implicit-def $r0, implicit-def dead $r1, implicit-def dead $r2, implicit-def dead $r3, implicit-def dead $r4, implicit-def dead $r5, implicit $r11
    ADJCALLSTACKUP 0, 0, implicit-def dead $r11, implicit $r11
    %5:gpr = COPY $r0
    %6:gpr = IMPLICIT_DEF
    $r0 = COPY %6:gpr
    RET implicit $r0
  
  # End machine code for function foo.
  
  # *** IR Dump After Process Implicit Definitions (processimpdefs) ***:
  # Machine code for function foo: IsSSA, TracksLiveness
  
  bb.0.entry:
    ADJCALLSTACKDOWN 0, 0, implicit-def dead $r11, implicit $r11
    JAL @bar, implicit-def $r0, implicit-def dead $r1, implicit-def dead $r2, implicit-def dead $r3, implicit-def dead $r4, implicit-def dead $r5, implicit $r11
    ADJCALLSTACKUP 0, 0, implicit-def dead $r11, implicit $r11
    RET implicit undef $r0
  
  # End machine code for function foo.
  
  1 error generated.
  
  0000000000000000 <foo>:
         0:	85 10 00 00 ff ff ff ff	call -0x1
         1:	95 00 00 00 00 00 00 00	exit

Note how `add` instruction disappeared, `processimpdefs` converted uses of `IMPLICIT_DEF` to `undef`, `undef` just peeks some physical register (implicitly defined r0 in this case). The final result looks confusing and will pass verification w/o issues. It looks like `getUNDEF()` would some times be confusing and sometimes lead to verifier errors (although one would need to track back these errors).


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  https://reviews.llvm.org/D156497/new/

https://reviews.llvm.org/D156497