Bharathi Seshadri via llvm-dev
2018-Jun-29 04:44 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
Hi,
I have come across a couple of cases where the code generated after
CodeGenPrepare pass has "br i1 false .." with both true and false
conditions preserved and this propagates further and remains the same
in the final assembly code/executable.
In CodeGenPrepare::runOnFunction, ConstantFoldTerminator (which
handles the br i1 false condition) is called only once and if after
the transformation of code by ConstantFoldTerminator() and
DeleteDeadBlock() we end up with code like "br i1 false", there is no
further opportunity to clean them up. So calling this code under
(!DisableBranchOpts) in a loop until no more transformations are made
fixes this issue. Is this reasonable ?
My simple fix (without any indentation changes) is:
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -316,7 +316,9 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
SunkAddrs.clear();
if (!DisableBranchOpts) {
+ MadeChange = true;
+ while (MadeChange) {
MadeChange = false;
SmallPtrSet<BasicBlock*, 8> WorkList;
for (BasicBlock &BB : F) {
@@ -352,6 +354,7 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
EverMadeChange |= MadeChange;
}
+ }
if (!DisableGCOpts) {
SmallVector<Instruction *, 2> Statepoints;
Testing the patch, I got a regression with
llvm/test/CodeGen/AMDGPU/nested-loop-conditions.ll. I am unsure if
this requires remastering the test case to adjust with the new results
or if this is a real issue. I don't have any expertise with GPU and
so any inputs in this regard would be very helpful.
Attached:
testcase.ll : Original test case for this issue
base.s and new.s: llc output for the failing case
(nested-loop-conditions.ll) before and after applying the above patch.
Thanks,
Bharathi
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; RUN: llc < %s | FileCheck %s
; CHECK-NOT: check_func
define void @copyfunc(i8* %py_src, i8* %py_grp) #0 {
entry:
%group17 = alloca [16 x i8], align 16
%source18 = alloca [16 x i8], align 16
%source219 = alloca [16 x i8], align 16
%source219.sub63 = bitcast [16 x i8]* %source219 to i8*
%source18.sub64 = bitcast [16 x i8]* %source18 to i8*
%group17.sub65 = bitcast [16 x i8]* %group17 to i8*
call void @llvm.lifetime.start(i64 16, i8* nonnull %group17.sub65) #5
call void @llvm.lifetime.start(i64 16, i8* nonnull %source18.sub64) #5
call void @llvm.lifetime.start(i64 16, i8* nonnull %source219.sub63) #5
%call = tail call i32 @getsize(i8* %py_grp) #5
%tobool = icmp ne i8* %py_src, null
%conv = sext i32 %call to i64
br i1 %tobool, label %if.then, label %if.end
if.then: ; preds = %entry
br i1 false, label %cond.true5.i, label %if.end.thread53
cond.true5.i: ; preds = %if.then
%0 = trunc i64 %conv to i32
%cmp6.i = icmp ugt i32 %0, 16
br i1 %cmp6.i, label %cond.true5.i26.thread, label %cond.true5.i26.thread59
cond.true5.i26.thread59: ; preds = %cond.true5.i
%1 = bitcast [16 x i8]* %source18 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %1, i8* nonnull %py_src, i64
%conv, i32 1, i1 false) #5
br label %cond.true8.i27
cond.true5.i26.thread: ; preds = %cond.true5.i
%call.i = tail call i32 @check_func() #5
br label %cond.false.i29
if.end.thread53: ; preds = %if.then
%2 = bitcast [16 x i8]* %source18 to i8*
%call10.i = call i8* @copy2(i64 16, i8* nonnull %2, i8* nonnull %py_src, i64
%conv) #5
br label %cond.false9.i31
if.end: ; preds = %entry
br i1 false, label %cond.true5.i26, label %cond.false9.i31
cond.true5.i26: ; preds = %if.end
%3 = trunc i64 %conv to i32
%cmp6.i25 = icmp ugt i32 %3, 16
br i1 %cmp6.i25, label %cond.false.i29, label %cond.true8.i27
cond.true8.i27: ; preds =
%cond.true5.i26.thread59, %cond.true5.i26
%4 = bitcast [16 x i8]* %group17 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %4, i8* %py_grp, i64 %conv,
i32 1, i1 false) #5
br label %copy.exit33
cond.false.i29: ; preds =
%cond.true5.i26.thread, %cond.true5.i26
%call.i28 = call i32 @check_func() #5
br label %copy.exit33
cond.false9.i31: ; preds = %if.end.thread53,
%if.end
%5 = bitcast [16 x i8]* %group17 to i8*
%call10.i30 = call i8* @copy2(i64 16, i8* nonnull %5, i8* %py_grp, i64 %conv)
#5
br label %copy.exit33
copy.exit33: ; preds = %cond.true8.i27,
%cond.false.i29, %cond.false9.i31
%.pre-phi49 = phi i1 [ true, %cond.true8.i27 ], [ true, %cond.false.i29 ], [
false, %cond.false9.i31 ]
%6 = icmp ne i8* %py_src, null
br i1 %6, label %cond.true.i38, label %if.end8
cond.true.i38: ; preds = %copy.exit33
br i1 %.pre-phi49, label %cond.true5.i40, label %cond.false9.i45
cond.true5.i40: ; preds = %cond.true.i38
%7 = trunc i64 %conv to i32
%cmp6.i39 = icmp ugt i32 %7, 16
br i1 %cmp6.i39, label %cond.false.i43, label %cond.true8.i41
cond.true8.i41: ; preds = %cond.true5.i40
%8 = bitcast [16 x i8]* %source219 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %8, i8* nonnull %py_src, i64
%conv, i32 1, i1 false) #5
br label %if.end8
cond.false.i43: ; preds = %cond.true5.i40
%call.i42 = call i32 @check_func() #5
br label %if.end8
cond.false9.i45: ; preds = %cond.true.i38
%9 = bitcast [16 x i8]* %source219 to i8*
%call10.i44 = call i8* @copy2(i64 16, i8* nonnull %9, i8* nonnull %py_src, i64
%conv) #5
br label %if.end8
if.end8: ; preds = %cond.false9.i45,
%cond.false.i43, %cond.true8.i41, %copy.exit33
%10 = bitcast [16 x i8]* %group17 to i8*
%11 = bitcast [16 x i8]* %source18 to i8*
%12 = bitcast [16 x i8]* %source219 to i8*
call void @llvm.lifetime.end(i64 16, i8* nonnull %12) #5
call void @llvm.lifetime.end(i64 16, i8* nonnull %11) #5
call void @llvm.lifetime.end(i64 16, i8* nonnull %10) #5
ret void
}
declare i32 @getsize(i8*) #2
declare i32 @check_func() #5
declare i8* @copy2(i64, i8*, i8*, i64) #2
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly,
i64, i32, i1) #1
declare void @llvm.lifetime.end(i64, i8* nocapture) #1
declare void @llvm.lifetime.start(i64, i8* nocapture) #1
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Friedman, Eli via llvm-dev
2018-Jun-29 19:20 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
On 6/28/2018 9:44 PM, Bharathi Seshadri via llvm-dev wrote:> Hi, > > I have come across a couple of cases where the code generated after > CodeGenPrepare pass has "br i1 false .." with both true and false > conditions preserved and this propagates further and remains the same > in the final assembly code/executable. > > In CodeGenPrepare::runOnFunction, ConstantFoldTerminator (which > handles the br i1 false condition) is called only once and if after > the transformation of code by ConstantFoldTerminator() and > DeleteDeadBlock() we end up with code like "br i1 false", there is no > further opportunity to clean them up. So calling this code under > (!DisableBranchOpts) in a loop until no more transformations are made > fixes this issue. Is this reasonable ?I would expect the precise case you're running into is rare: the second iteration of the loop does nothing useful unless the IR specifically has an i1 phi node in a block whose predecessors were erased. And the default optimization pipeline runs SimplifyCFG at the very end, which is close to CodeGenPrepare, so the CFG simplification will usually be a no-op anyway. We really shouldn't be doing this sort of folding in CodeGenPrepare in the first place. It looks like it was added to work around the fact that we we lower llvm.objectsize later than we should. -Eli -- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
George Burgess IV via llvm-dev
2018-Jun-29 22:25 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
> we lower llvm.objectsize later than we shouldIs there a well-accepted best (or even just better) place to lower objectsize? I ask because I sorta fear that these kinds of problems will become more pronounced as llvm.is.constant, which is also lowered in CGP, gains popularity. (To be clear, I think it totally makes sense to lower is.constant and objectsize in the same place. I'm just saying that if the ideal piece of code to do that isn't CGP, ...) On Fri, Jun 29, 2018 at 12:21 PM Friedman, Eli via llvm-dev < llvm-dev at lists.llvm.org> wrote:> On 6/28/2018 9:44 PM, Bharathi Seshadri via llvm-dev wrote: > > Hi, > > > > I have come across a couple of cases where the code generated after > > CodeGenPrepare pass has "br i1 false .." with both true and false > > conditions preserved and this propagates further and remains the same > > in the final assembly code/executable. > > > > In CodeGenPrepare::runOnFunction, ConstantFoldTerminator (which > > handles the br i1 false condition) is called only once and if after > > the transformation of code by ConstantFoldTerminator() and > > DeleteDeadBlock() we end up with code like "br i1 false", there is no > > further opportunity to clean them up. So calling this code under > > (!DisableBranchOpts) in a loop until no more transformations are made > > fixes this issue. Is this reasonable ? > > I would expect the precise case you're running into is rare: the second > iteration of the loop does nothing useful unless the IR specifically has > an i1 phi node in a block whose predecessors were erased. And the > default optimization pipeline runs SimplifyCFG at the very end, which is > close to CodeGenPrepare, so the CFG simplification will usually be a > no-op anyway. > > We really shouldn't be doing this sort of folding in CodeGenPrepare in > the first place. It looks like it was added to work around the fact > that we we lower llvm.objectsize later than we should. > > -Eli > > -- > Employee of Qualcomm Innovation Center, Inc. > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux > Foundation Collaborative Project > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180629/0284ede1/attachment.html>