[PATCH] D95543: [GVN] Clobber partially aliased loads.

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/test/Transforms/GVN/PRE/rle.ll:736
+; Cross block partial alias case with phi translation.
+; FIXME: Currently we fail to clobber both partially aliased cases of load.
+define i32 @load_load_partial_alias_cross_block_phi_trans(i8* %P) nounwind ssp {
----------------
I believe for phi translation to work you need to have the translated geps in the predecessor. This test case worked for me:
```
define i32 @load_load_partial_alias_cross_block_phi_trans(i8* %P) nounwind {
; LE-LABEL: @load_load_partial_alias_cross_block_phi_trans(
; LE-NEXT:  entry:
; LE-NEXT:    [[XX:%.*]] = bitcast i8* [[P:%.*]] to i32*
; LE-NEXT:    [[X1:%.*]] = load i32, i32* [[XX]], align 4
; LE-NEXT:    [[CMP:%.*]] = icmp eq i32 [[X1]], 127
; LE-NEXT:    [[TMP0:%.*]] = lshr i32 [[X1]], 16
; LE-NEXT:    [[TMP1:%.*]] = trunc i32 [[TMP0]] to i8
; LE-NEXT:    [[TMP2:%.*]] = lshr i32 [[X1]], 8
; LE-NEXT:    [[TMP3:%.*]] = trunc i32 [[TMP2]] to i8
; LE-NEXT:    br i1 [[CMP]], label [[IF:%.*]], label [[ELSE:%.*]]
; LE:       if:
; LE-NEXT:    br label [[JOIN:%.*]]
; LE:       else: 
; LE-NEXT:    br label [[JOIN]]
; LE:       join: 
; LE-NEXT:    [[TMP5:%.*]] = phi i8 [ [[TMP3]], [[IF]] ], [ [[TMP1]], [[ELSE]] ]
; LE-NEXT:    [[CONV6:%.*]] = zext i8 [[TMP5]] to i32
; LE-NEXT:    ret i32 [[CONV6]]
; LE:       if.end:
; LE-NEXT:    ret i32 52 
; 
; BE-LABEL: @load_load_partial_alias_cross_block2(
; BE-NEXT:  entry:
; BE-NEXT:    [[XX:%.*]] = bitcast i8* [[P:%.*]] to i32*
; BE-NEXT:    [[X1:%.*]] = load i32, i32* [[XX]], align 4
; BE-NEXT:    [[CMP:%.*]] = icmp eq i32 [[X1]], 127
; BE-NEXT:    [[TMP0:%.*]] = lshr i32 [[X1]], 8
; BE-NEXT:    [[TMP1:%.*]] = trunc i32 [[TMP0]] to i8
; BE-NEXT:    [[TMP2:%.*]] = lshr i32 [[X1]], 16
; BE-NEXT:    [[TMP3:%.*]] = trunc i32 [[TMP2]] to i8
; BE-NEXT:    br i1 [[CMP]], label [[IF:%.*]], label [[ELSE:%.*]]
; BE:       if:
; BE-NEXT:    br label [[JOIN:%.*]]
; BE:       else: 
; BE-NEXT:    br label [[JOIN]]
; BE:       join: 
; BE-NEXT:    [[TMP5:%.*]] = phi i8 [ [[TMP3]], [[IF]] ], [ [[TMP1]], [[ELSE]] ]
; BE-NEXT:    [[CONV6:%.*]] = zext i8 [[TMP5]] to i32
; BE-NEXT:    ret i32 [[CONV6]]
; BE:       if.end:
; BE-NEXT:    ret i32 52
; 
entry:
  %xx = bitcast i8* %P to i32*
  %x1 = load i32, i32* %xx, align 4
  %cmp = icmp eq i32 %x1, 127
  br i1 %cmp, label %if, label %else

if:
  %arrayidx.if = getelementptr inbounds i8, i8* %P, i64 1
  br label %join

else:
  %arrayidx.else = getelementptr inbounds i8, i8* %P, i64 2
  br label %join
  
join: 
  %idx = phi i64 [ 1, %if ], [ 2, %else ]
  %arrayidx4 = getelementptr inbounds i8, i8* %P, i64 %idx
  %tmp5 = load i8, i8* %arrayidx4, align 1
  %conv6 = zext i8 %tmp5 to i32
  ret i32 %conv6

if.end:
  ret i32 52
}
```

I think the actually problematic case probably has something to do with phi translation in loops (where you translate into the same block, but in a different iteration). I'll try to play with that tomorrow, but if I don't find anything let's just land this and see if any miscompiles turn up :)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D95543/new/

https://reviews.llvm.org/D95543