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llvm-mirror/test/Transforms/InstCombine/bitcast-bigendian.ll
Juneyoung Lee a7f581c4d0 [InstCombine] use poison as placeholder for undemanded elems 
Currently undef is used as a don’t-care vector when constructing a vector using a series of insertelement.
However, this is problematic because undef isn’t undefined enough.
Especially, a sequence of insertelement can be optimized to shufflevector, but using undef as its placeholder makes shufflevector a poison-blocking instruction because undef cannot be optimized to poison.
This makes a few straightforward optimizations incorrect, such as:

```
;  https://bugs.llvm.org/show_bug.cgi?id=44185

define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) {
  %xv = insertelement <4 x float> %q, float %x, i32 2
  %r = shufflevector <4 x float> %y, <4 x float> %xv, <4 x i32> { 0, 6, 2, undef }
  ret <4 x float> %r ; %r[3] is undef
}
=>
define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) {
  %r = insertelement <4 x float> %y, float %x, i32 1
  ret <4 x float> %r ; %r[3] = %y[3], incorrect if %y[3] = poison
}

Transformation doesn't verify!
ERROR: Target is more poisonous than source
```

I’d like to suggest
1. Using poison as insertelement’s placeholder value (IRBuilder::CreateVectorSplat should be patched too)
2. Updating shufflevector’s semantics to return poison element if mask is undef

Note that poison is currently lowered into UNDEF in SelDag, so codegen part is okay.
m_Undef() matches PoisonValue as well, so existing optimizations will still fire.

The only concern is hidden miscompilations that will go incorrect when poison constant is given.
A conservative way is copying all tests having `insertelement undef` & replacing it with `insertelement poison` & run Alive2 on it, but it will create many tests and people won’t like it. :(

Instead, I’ll simply locally maintain the tests and run Alive2.
If there is any bug found, I’ll report it.

Relevant links: https://bugs.llvm.org/show_bug.cgi?id=43958 , http://lists.llvm.org/pipermail/llvm-dev/2019-November/137242.html

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D93586
2020-12-28 08:58:15 +09:00

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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s -instcombine -S | FileCheck %s
target datalayout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-f128:128:128-v128:128:128-n32:64"
target triple = "powerpc64-unknown-linux-gnu"
; These tests are extracted from bitcast.ll.
; Verify that they also work correctly on big-endian targets.
define float @test2(<2 x float> %A, <2 x i32> %B) {
; CHECK-LABEL: @test2(
; CHECK-NEXT: [[TMP24:%.*]] = extractelement <2 x float> [[A:%.*]], i32 1
; CHECK-NEXT: [[BC:%.*]] = bitcast <2 x i32> [[B:%.*]] to <2 x float>
; CHECK-NEXT: [[TMP4:%.*]] = extractelement <2 x float> [[BC]], i32 1
; CHECK-NEXT: [[ADD:%.*]] = fadd float [[TMP24]], [[TMP4]]
; CHECK-NEXT: ret float [[ADD]]
;
%tmp28 = bitcast <2 x float> %A to i64
%tmp23 = trunc i64 %tmp28 to i32
%tmp24 = bitcast i32 %tmp23 to float
%tmp = bitcast <2 x i32> %B to i64
%tmp2 = trunc i64 %tmp to i32
%tmp4 = bitcast i32 %tmp2 to float
%add = fadd float %tmp24, %tmp4
ret float %add
}
define float @test3(<2 x float> %A, <2 x i64> %B) {
; CHECK-LABEL: @test3(
; CHECK-NEXT: [[TMP24:%.*]] = extractelement <2 x float> [[A:%.*]], i32 0
; CHECK-NEXT: [[BC2:%.*]] = bitcast <2 x i64> [[B:%.*]] to <4 x float>
; CHECK-NEXT: [[TMP4:%.*]] = extractelement <4 x float> [[BC2]], i32 1
; CHECK-NEXT: [[ADD:%.*]] = fadd float [[TMP24]], [[TMP4]]
; CHECK-NEXT: ret float [[ADD]]
;
%tmp28 = bitcast <2 x float> %A to i64
%tmp29 = lshr i64 %tmp28, 32
%tmp23 = trunc i64 %tmp29 to i32
%tmp24 = bitcast i32 %tmp23 to float
%tmp = bitcast <2 x i64> %B to i128
%tmp1 = lshr i128 %tmp, 64
%tmp2 = trunc i128 %tmp1 to i32
%tmp4 = bitcast i32 %tmp2 to float
%add = fadd float %tmp24, %tmp4
ret float %add
}
define <2 x i32> @test4(i32 %A, i32 %B){
; CHECK-LABEL: @test4(
; CHECK-NEXT: [[TMP1:%.*]] = insertelement <2 x i32> poison, i32 [[B:%.*]], i32 0
; CHECK-NEXT: [[TMP2:%.*]] = insertelement <2 x i32> [[TMP1]], i32 [[A:%.*]], i32 1
; CHECK-NEXT: ret <2 x i32> [[TMP2]]
;
%tmp38 = zext i32 %A to i64
%tmp32 = zext i32 %B to i64
%tmp33 = shl i64 %tmp32, 32
%ins35 = or i64 %tmp33, %tmp38
%tmp43 = bitcast i64 %ins35 to <2 x i32>
ret <2 x i32> %tmp43
}
define <2 x float> @test5(float %A, float %B) {
; CHECK-LABEL: @test5(
; CHECK-NEXT: [[TMP1:%.*]] = insertelement <2 x float> poison, float [[B:%.*]], i32 0
; CHECK-NEXT: [[TMP2:%.*]] = insertelement <2 x float> [[TMP1]], float [[A:%.*]], i32 1
; CHECK-NEXT: ret <2 x float> [[TMP2]]
;
%tmp37 = bitcast float %A to i32
%tmp38 = zext i32 %tmp37 to i64
%tmp31 = bitcast float %B to i32
%tmp32 = zext i32 %tmp31 to i64
%tmp33 = shl i64 %tmp32, 32
%ins35 = or i64 %tmp33, %tmp38
%tmp43 = bitcast i64 %ins35 to <2 x float>
ret <2 x float> %tmp43
}
define <2 x float> @test6(float %A){
; CHECK-LABEL: @test6(
; CHECK-NEXT: [[TMP1:%.*]] = insertelement <2 x float> <float poison, float 4.200000e+01>, float [[A:%.*]], i32 0
; CHECK-NEXT: ret <2 x float> [[TMP1]]
;
%tmp23 = bitcast float %A to i32
%tmp24 = zext i32 %tmp23 to i64
%tmp25 = shl i64 %tmp24, 32
%mask20 = or i64 %tmp25, 1109917696
%tmp35 = bitcast i64 %mask20 to <2 x float>
ret <2 x float> %tmp35
}
; No change. Bitcasts are canonicalized above bitwise logic.
define <2 x i32> @xor_bitcast_vec_to_vec(<1 x i64> %a) {
; CHECK-LABEL: @xor_bitcast_vec_to_vec(
; CHECK-NEXT: [[T1:%.*]] = bitcast <1 x i64> [[A:%.*]] to <2 x i32>
; CHECK-NEXT: [[T2:%.*]] = xor <2 x i32> [[T1]], <i32 1, i32 2>
; CHECK-NEXT: ret <2 x i32> [[T2]]
;
%t1 = bitcast <1 x i64> %a to <2 x i32>
%t2 = xor <2 x i32> <i32 1, i32 2>, %t1
ret <2 x i32> %t2
}
; No change. Bitcasts are canonicalized above bitwise logic.
define i64 @and_bitcast_vec_to_int(<2 x i32> %a) {
; CHECK-LABEL: @and_bitcast_vec_to_int(
; CHECK-NEXT: [[T1:%.*]] = bitcast <2 x i32> [[A:%.*]] to i64
; CHECK-NEXT: [[T2:%.*]] = and i64 [[T1]], 3
; CHECK-NEXT: ret i64 [[T2]]
;
%t1 = bitcast <2 x i32> %a to i64
%t2 = and i64 %t1, 3
ret i64 %t2
}
; No change. Bitcasts are canonicalized above bitwise logic.
define <2 x i32> @or_bitcast_int_to_vec(i64 %a) {
; CHECK-LABEL: @or_bitcast_int_to_vec(
; CHECK-NEXT: [[T1:%.*]] = bitcast i64 [[A:%.*]] to <2 x i32>
; CHECK-NEXT: [[T2:%.*]] = or <2 x i32> [[T1]], <i32 1, i32 2>
; CHECK-NEXT: ret <2 x i32> [[T2]]
;
%t1 = bitcast i64 %a to <2 x i32>
%t2 = or <2 x i32> %t1, <i32 1, i32 2>
ret <2 x i32> %t2
}
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