[llvm-dev] how experimental are the llvm.experimental.vector.reduce.* functions?

Andrew Kelley via llvm-dev
Sat, 09 Feb 2019 07:02:49 -0000

I'm interested in using @llvm.experimental.vector.reduce.smax/umax to
implement runtime overflow checking for vectors. Here's an example
checked addition, without vectors, and then I'll follow the example with
what I would do for checked addition with vectors.

Frontend code (zig):

export fn entry() void {
    var a: i32 = 1;
    var b: i32 = 2;
    var x = a + b;
}

LLVM IR code:

define void @entry() #2 !dbg !41 {
Entry:
  %a = alloca i32, align 4
  %b = alloca i32, align 4
  %x = alloca i32, align 4
  store i32 1, i32* %a, align 4, !dbg !52
  call void @llvm.dbg.declare(metadata i32* %a, metadata !45, metadata
!DIExpression()), !dbg !52
  store i32 2, i32* %b, align 4, !dbg !53
  call void @llvm.dbg.declare(metadata i32* %b, metadata !48, metadata
!DIExpression()), !dbg !53
  %0 = load i32, i32* %a, align 4, !dbg !54
  %1 = load i32, i32* %b, align 4, !dbg !55
  %2 = call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %0, i32 %1),
!dbg !56
  %3 = extractvalue { i32, i1 } %2, 0, !dbg !56
  %4 = extractvalue { i32, i1 } %2, 1, !dbg !56
  br i1 %4, label %OverflowFail, label %OverflowOk, !dbg !56

OverflowFail:                                     ; preds = %Entry
  tail call fastcc void @panic(%"[]u8"* @2, %StackTrace* null), !dbg
!56
  unreachable, !dbg !56

OverflowOk:                                       ; preds = %Entry
  store i32 %3, i32* %x, align 4, !dbg !57
  call void @llvm.dbg.declare(metadata i32* %x, metadata !50, metadata
!DIExpression()), !dbg !57
  ret void, !dbg !58
}

You can see this takes advantage of @llvm.sadd.with.overflow, which is
not available with vectors. So here is a different approach (pseudocode):

%a_zext = zext %a to i33 # 1 more bit
%b_zext = zext %b to i33 # 1 more bit
%result_zext = add %a_zext, %b_zext
%max_result = @llvm.experimental.vector.reduce.umax(%result_zext)
%overflow = icmp %max_result > @max_i32_value
%result = trunc %result_zext to i32

You can imagine how this would work for signed integers, replacing zext
with sext and umax with smax.

This depends on an "experimental" API. Can anyone advise on depending
on
this API? Is it a bad idea? Is it about to be promoted to
non-experimental soon? Can anyone advise on how to best achieve my goal?

Kind regards,
Andrew

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