[PATCH] D90529: Allow nonnull/align attribute to accept poison

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

In D90529#2613303 <https://reviews.llvm.org/D90529#2613303>, @Tyker wrote:

> In D90529#2612794 <https://reviews.llvm.org/D90529#2612794>, @aqjune wrote:
>
>> A fix in AssumeBundleBuilder to make it comply LangRef: D98228 <https://reviews.llvm.org/D98228>
>>
>> BTW, I found that "align" can take two operands: `"align"(i8* ptr, i64 a, i64 b)` What is the meaning of the second index (`b`)?
>
> the meaning of the second index is the offset of the alignment. so with `"align"(i8* ptr, i64 16, i64 12)`, ptr is aligned on 4 but ptr + 4 is aligned on 16.
> I introduced this to support __builtin_assume_aligned which has similar semantics.

Hi, thanks for the info.

But I think an example in Transforms/AlignmentFromAssumptions/simple.ll conflicts with your definition. It has:

  28 define i32 @foo2a(i32* nocapture %a) nounwind uwtable readonly {
  29 entry:
  30   tail call void @llvm.assume(i1 true) ["align"(i32* %a, i32 32, i32 28)]
  31   %arrayidx = getelementptr inbounds i32, i32* %a, i64 -1
  32   %0 = load i32, i32* %arrayidx, align 4
  33   ret i32 %0
  34 
  35 ; CHECK-LABEL: @foo2a
  36 ; CHECK: load i32, i32* {{[^,]+}}, align 32
  37 ; CHECK: ret i32
  38 }

`"align"(i32* %a, i32 32, i32 28)` means `%a + 4` is 32-bytes aligned, IIUC. Then, `%a - 4` cannot be 32-bytes aligned, is it?

>From https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html , the definition you've explained seems to be correct because it is saying

  void *x = __builtin_assume_aligned (arg, 32, 8);
  means that the compiler can assume for x, set to arg, that (char *) x - 8 is 32-byte aligned.

Should we specify this in LangRef as well to make its definition explicit?


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