[llvm] [BOLT] Avoid reference updates for non-JT symbol operands (PR #88838)

[via llvm-commits]

linsinan1995 wrote:

> Thank you for the fix. How did you discover the problem?

Hi @maksfb , I found this problem in an internal testsuite built with clang/llvm toolchain. I did some investigation and saw that some SCEV-based optimizations like loop-reduce and slsr in LLVM can generate this kind of pattern. 

LLVM IR log:
```
define dso_local i32 @main(i32 noundef %0, ptr nocapture noundef readnone %1) local_unnamed_addr #0 {
  ...
  br label %5

5:                                                ; preds = %5, %2
  %6 = phi i64 [ 0, %2 ], [ %62, %5 ]
  %7 = phi i32 [ %4, %2 ], [ %61, %5 ]
  ...
  %36 = getelementptr inbounds [8 x [2 x i8]], ptr @c, i64 0, i64 %6, i64 1
  %37 = load i8, ptr %36, align 1, !tbaa !8
  %38 = sext i8 %37 to i32
  ...
  %50 = ashr i32 %49, 8
  %62 = add nuw nsw i64 %6, 1
  %63 = icmp eq i64 %62, 8
  br i1 %63, label %64, label %5, !llvm.loop !9

*** IR Dump After Loop Strength Reduction (loop-reduce) ***
; Preheader:
  ...
  br label %5

; Loop:
5:                                                ; preds = %5, %2
  %6 = phi i64 [ %64, %5 ], [ -16, %2 ]
  ...
  %37 = getelementptr i8, ptr @c, i64 %6
  %38 = getelementptr i8, ptr %37, i64 17
  %39 = load i8, ptr %38, align 1, !tbaa !8
  %40 = sext i8 %39 to i32
  %41 = xor i32 %35, %40
  %42 = sext i32 %41 to i64
```

in this case, llvm optimize `getelementptr inbounds [8 x [2 x i8]], ptr @c, i64 0, i64 %6, i64 1` into two simpler operations, `%37 = getelementptr i8, ptr @c, i64 %6` and `getelementptr i8, ptr %37, i64 17`. 

after removing PHI
```
  %7:gr64 = MOV64ri32 -16
  %0:gr32 = MOV32rm $rip, 1, $noreg, @b, $noreg :: (dereferenceable load (s32) from @b, !tbaa !4)
  %60:gr64 = COPY killed %7:gr64
  %61:gr32 = COPY killed %0:gr32

bb.1 (%ir-block.5):
; predecessors: %bb.0, %bb.1
  successors: %bb.2(0x04000000), %bb.1(0x7c000000); %bb.2(3.12%), %bb.1(96.88%)

  %2:gr32 = COPY killed %61:gr32
  %1:gr64 = COPY killed %60:gr64
  ...
  %41:gr32 = MOVSX32rm8 %1:gr64, 1, $noreg, @c + 17, $noreg :: (invariant load (s8) from %ir.38, !tbaa !8)
  %42:gr32 = XOR32rr killed %39:gr32(tied-def 0), killed %41:gr32, implicit-def dead $eflags
  ...
  %60:gr64 = COPY killed %4:gr64
  %61:gr32 = COPY %3:gr32
  JCC_1 %bb.1, 5, implicit killed $eflags
  JMP_1 %bb.2
```


Then we have
```
movq	$-0x10, %rax
movl	c+17(%rax), %edx
```

https://github.com/llvm/llvm-project/pull/88838