[llvm-dev] Wrong code bug after GVN/PRE?
Mikael Holmén via llvm-dev
llvm-dev at lists.llvm.org
Fri Jan 13 00:31:21 PST 2017
Hi,
I've stumbled upon a case where I think gvn does a bad (wrong)
optimization. It's a bit messy to debug though so I'm not sure if I
should just write a PR about it a let someone who knows the code look at
it instead.
Anyway, for the bug to trigger I need to run the following passes in the
same opt invocation:
-sroa -instcombine -simplifycfg -instcombine -gvn
The problem seems to be that GVN::PerformLoadPRE triggers and I see a
GVN REMOVING PRE LOAD: %_tmp79 = load i16, i16* %_tmp78, align 2
printout.
If I instead first run
-sroa -instcombine -simplifycfg -instcombine
and then
-gvn
I don't get the
GVN REMOVING PRE LOAD
printout, and the resulting code looks ok to me.
Is this expected? Should the output differ in these two cases?
The input to gvn looks the same when running all passes and just gvn,
but I see a difference in how GVN::processNonLocalLoad(LoadInst *LI)
behaves:
I get different results from
// Step 1: Find the non-local dependencies of the load.
LoadDepVect Deps;
MD->getNonLocalPointerDependency(LI, Deps);
So we get different results from MemoryDependenceResults when invoking
gvn alone or after a bunch of other passes.
Is this expected or not?
I tried to dig into
MemoryDependenceResults::getNonLocalPointerDependency but I have to say
I'm quite lost.
I ran some git bisect and as far as I can tell it starts going wrong
with commit
[SimplifyCFG] Change the algorithm in SinkThenElseCodeToEnd
2016-09-01, but that commit doesn't change GVN/MemoryDependenceResults
so I suppose it just changes the input to GVN so the problem suddenly
surfaces.
Finally, the problem that I see is:
In the input we have something like
for (int step1 = 0; step1 < LOOP_AMOUNT; step1++)
{
lb[step1] = step1 + 7;
ub[step1] = (step1 == 0 ? 10: ub[step1-1]*10);
switch(lb[step1]) {
case 7: CVAL_VERIFY(step1 == 0);
CVAL_VERIFY(ub[step1] == 10);
__label(511);
break;
case 8: CVAL_VERIFY(step1 == 1);
CVAL_VERIFY(ub[step1] == 100);
__label(512);
break;
case 9: CVAL_VERIFY(step1 == 2);
CVAL_VERIFY(ub[step1] == 1000);
__label(513);
break;
default:;
}
[...]
}
int i, j;
for ( j = 10, i = 0; j < 101; j=j*10, i++ )
{
CVAL_VERIFY(ub[i] == j);
}
So we have two loops. In the first one the array ub is written (low
index first, high index last), and then in the second loop the content
of ub is checked (low index first, high index last).
After gvn this is changed into something like
int tmp;
for (int step1 = 0; step1 < LOOP_AMOUNT; step1++)
{
lb[step1] = step1 + 7;
ub[step1] = (step1 == 0 ? 10: ub[step1-1]*10);
switch(lb[step1]) {
case 7: CVAL_VERIFY(step1 == 0);
tmp = ub[step1];
CVAL_VERIFY(tmp == 10);
__label(511);
break;
case 8: CVAL_VERIFY(step1 == 1);
tmp = ub[step1];
CVAL_VERIFY(tmp == 100);
__label(512);
break;
case 9: CVAL_VERIFY(step1 == 2);
tmp = ub[step1];
CVAL_VERIFY(tmp == 1000);
__label(513);
break;
default:;
}
[...]
}
int i, j;
for ( j = 10, i = 0; j < 101; j=j*10, i++ )
{
CVAL_VERIFY((i == 0 ? tmp : ub[i]) == j);
}
Note the introduced variable tmp.
Also note that with this rewrite, after the first loop tmp will contain
the value of the element at the highest index in ub, and then we use tmp
in the first round of the second loop, but there we expect the value of
the lowest index element in ub.
Any ideas? Should I file a PR?
Thanks,
Mikael
-------------- next part --------------
; llvm assembler.
target datalayout = "p:16:16"
%int4 = type i16
%int5 = type i16
%ptr7 = type %int4 *
%arr18 = type [4 x %int4]
%ptr20 = type %arr18 *
%arr22 = type [16 x %int4]
%ptr24 = type %arr22 *
%ptr26 = type %ptr20 *
declare void @CVAL_VERIFY_FUNC (%int4 %val.6.par) optsize minsize noinline;
define %int4 @main () optsize minsize {
%step1.7 = alloca %int4
%i.8 = alloca %int4
%j.9 = alloca %int4
%globcse1.10 = alloca %int4
%cach1.11 = alloca %int4
%liv1.12 = alloca %ptr20
%liv2.13 = alloca %ptr20
%liv3.14 = alloca %ptr20
%lb.15 = alloca %arr18
%ub.16 = alloca %arr18
store %ptr20 %lb.15, %ptr26 %liv2.13
store %ptr20 %ub.16, %ptr26 %liv1.12
store %int4 0, %ptr7 %step1.7
br label %bb1
bb1:
%_tmp5 = load %int4, %ptr7 %step1.7
%_tmp6 = add %int4 %_tmp5, 7
store %int4 %_tmp6, %ptr7 %cach1.11
%_tmp7 = load %int4, %ptr7 %cach1.11
%_tmp8 = load %ptr20, %ptr26 %liv2.13
%_tmp9 = load %int4, %ptr7 %step1.7
%_tmp10 = sext %int4 %_tmp9 to i64
%_tmp11 = getelementptr %arr18, %ptr20 %_tmp8, i16 0, i64 %_tmp10
store %int4 %_tmp7, %ptr7 %_tmp11
%_tmp12 = load %int4, %ptr7 %step1.7
%_tmp13 = icmp eq %int4 %_tmp12, 0
br i1 %_tmp13, label %bb3, label %bb4
bb3:
%_tmp14 = load %ptr20, %ptr26 %liv1.12
%_tmp15 = bitcast %ptr20 %_tmp14 to %ptr7
store %int4 10, %ptr7 %_tmp15
br label %bb5
bb4:
%_tmp16 = load %ptr20, %ptr26 %liv1.12
%_tmp17 = load %int4, %ptr7 %step1.7
%_tmp18 = sub %int4 %_tmp17, 1
%_tmp19 = sext %int4 %_tmp18 to i64
%_tmp20 = getelementptr %arr18, %ptr20 %_tmp16, i16 0, i64 %_tmp19
%_tmp21 = load %int4, %ptr7 %_tmp20
%_tmp22 = mul %int4 %_tmp21, 10
%_tmp23 = load %ptr20, %ptr26 %liv1.12
%_tmp24 = load %int4, %ptr7 %step1.7
%_tmp25 = sext %int4 %_tmp24 to i64
%_tmp26 = getelementptr %arr18, %ptr20 %_tmp23, i16 0, i64 %_tmp25
store %int4 %_tmp22, %ptr7 %_tmp26
br label %bb5
bb5:
%_tmp27 = load %int4, %ptr7 %cach1.11
%_tmp28 = icmp eq %int4 %_tmp27, 7
br i1 %_tmp28, label %bb6, label %bb_usw2
bb6:
%_tmp29 = load %int4, %ptr7 %step1.7
%_tmp30 = icmp eq %int4 %_tmp29, 0
%_tmp31 = select i1 %_tmp30, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp31)
%_tmp32 = load %ptr20, %ptr26 %liv1.12
%_tmp33 = load %int4, %ptr7 %step1.7
%_tmp34 = sext %int4 %_tmp33 to i64
%_tmp35 = getelementptr %arr18, %ptr20 %_tmp32, i16 0, i64 %_tmp34
%_tmp36 = load %int4, %ptr7 %_tmp35
%_tmp37 = icmp eq %int4 %_tmp36, 10
%_tmp38 = select i1 %_tmp37, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp38)
br label %bb9
bb_usw2:
%_tmp39 = load %int4, %ptr7 %cach1.11
%_tmp40 = icmp eq %int4 %_tmp39, 8
br i1 %_tmp40, label %bb7, label %bb8
bb7:
%_tmp41 = load %int4, %ptr7 %step1.7
%_tmp42 = icmp eq %int4 %_tmp41, 1
%_tmp43 = select i1 %_tmp42, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp43)
%_tmp44 = load %ptr20, %ptr26 %liv1.12
%_tmp45 = load %int4, %ptr7 %step1.7
%_tmp46 = sext %int4 %_tmp45 to i64
%_tmp47 = getelementptr %arr18, %ptr20 %_tmp44, i16 0, i64 %_tmp46
%_tmp48 = load %int4, %ptr7 %_tmp47
%_tmp49 = icmp eq %int4 %_tmp48, 100
%_tmp50 = select i1 %_tmp49, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp50)
br label %bb9
bb8:
%_tmp51 = load %int4, %ptr7 %step1.7
%_tmp52 = icmp eq %int4 %_tmp51, 2
%_tmp53 = select i1 %_tmp52, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp53)
%_tmp54 = load %ptr20, %ptr26 %liv1.12
%_tmp55 = load %int4, %ptr7 %step1.7
%_tmp56 = sext %int4 %_tmp55 to i64
%_tmp57 = getelementptr %arr18, %ptr20 %_tmp54, i16 0, i64 %_tmp56
%_tmp58 = load %int4, %ptr7 %_tmp57
%_tmp59 = icmp eq %int4 %_tmp58, 1000
%_tmp60 = select i1 %_tmp59, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp60)
br label %bb9
bb9:
%_tmp61 = load %int4, %ptr7 %cach1.11
%_tmp62 = icmp eq %int4 %_tmp61, 8
br i1 %_tmp62, label %bb10, label %bb_usw4
bb10:
%_tmp63 = load %int4, %ptr7 %step1.7
%_tmp64 = icmp eq %int4 %_tmp63, 1
%_tmp65 = select i1 %_tmp64, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp65)
br label %bb12
bb_usw4:
%_tmp66 = load %int4, %ptr7 %cach1.11
%_tmp67 = icmp eq %int4 %_tmp66, 9
br i1 %_tmp67, label %bb11, label %bb12
bb11:
%_tmp68 = load %int4, %ptr7 %step1.7
%_tmp69 = icmp eq %int4 %_tmp68, 2
%_tmp70 = select i1 %_tmp69, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp70)
br label %bb12
bb12:
%_tmp71 = load %int4, %ptr7 %step1.7
%_tmp72 = add %int4 %_tmp71, 1
store %int4 %_tmp72, %ptr7 %step1.7
%_tmp73 = load %int4, %ptr7 %step1.7
%_tmp74 = icmp slt %int4 %_tmp73, 3
br i1 %_tmp74, label %bb1, label %bb13
bb13:
store %int4 10, %ptr7 %j.9
store %ptr20 %ub.16, %ptr26 %liv3.14
store %int4 0, %ptr7 %i.8
br label %bb14
bb14:
%_tmp75 = load %ptr20, %ptr26 %liv3.14
%_tmp76 = load %int4, %ptr7 %i.8
%_tmp77 = sext %int4 %_tmp76 to i64
%_tmp78 = getelementptr %arr18, %ptr20 %_tmp75, i16 0, i64 %_tmp77
%_tmp79 = load %int4, %ptr7 %_tmp78
store %int4 %_tmp79, %ptr7 %globcse1.10
%_tmp84 = load %int4, %ptr7 %globcse1.10
%_tmp85 = load %int4, %ptr7 %j.9
%_tmp86 = icmp eq %int4 %_tmp84, %_tmp85
%_tmp87 = select i1 %_tmp86, %int4 1, %int4 0
call void @CVAL_VERIFY_FUNC(%int4 %_tmp87)
%_tmp88 = load %int4, %ptr7 %j.9
%_tmp89 = mul %int4 %_tmp88, 10
store %int4 %_tmp89, %ptr7 %j.9
%_tmp90 = load %int4, %ptr7 %i.8
%_tmp91 = add %int4 %_tmp90, 1
store %int4 %_tmp91, %ptr7 %i.8
%_tmp92 = load %int4, %ptr7 %j.9
%_tmp93 = icmp slt %int4 %_tmp92, 101
br i1 %_tmp93, label %bb14, label %bb16
bb16:
br label %bb17
bb17:
ret %int4 zeroinitializer
}
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