[llvm] r225034 - InstCombine: try to transform A-B < 0 into A < B
Kuperstein, Michael M
michael.m.kuperstein at intel.com
Wed Jan 14 05:25:11 PST 2015
Hi David,
It turns out this is causing quite a lot of performance regressions.
One of the issues is that this messes up IndVar simplification.
For this code:
int foo (int *p, int M, int N, int K)
{
int ret = 0;
for (int i = 0; i < M-N; i++)
ret += p[i * K];
return ret;
}
What we get coming into IndVar Simplify before the patch is:
define i32 @foo(i32* nocapture readonly %p, i32 %M, i32 %N, i32 %K) #0 {
entry:
%sub = sub nsw i32 %M, %N
%cmp4 = icmp sgt i32 %sub, 0
br i1 %cmp4, label %for.body.lr.ph, label %for.end
for.body.lr.ph: ; preds = %entry
br label %for.body
for.body: ; preds = %for.body.lr.ph, %for.body
%i.06 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
%ret.05 = phi i32 [ 0, %for.body.lr.ph ], [ %add, %for.body ]
%mul = mul nsw i32 %i.06, %K
%arrayidx = getelementptr inbounds i32* %p, i32 %mul
%0 = load i32* %arrayidx, align 4, !tbaa !1
%add = add nsw i32 %0, %ret.05
%inc = add nsw i32 %i.06, 1
%cmp = icmp slt i32 %inc, %sub
br i1 %cmp, label %for.body, label %for.cond.for.end_crit_edge
for.cond.for.end_crit_edge: ; preds = %for.body
%add.lcssa = phi i32 [ %add, %for.body ]
br label %for.end
for.end: ; preds = %for.cond.for.end_crit_edge, %entry
%ret.0.lcssa = phi i32 [ %add.lcssa, %for.cond.for.end_crit_edge ], [ 0, %entry ]
ret i32 %ret.0.lcssa
}
IndVar Simplify manages to compute the SCEV for the induction variable:
INDVARS: Rewriting loop exit condition to:
LHS: %i.06 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
op: !=
RHS: %1 = sub i32 %0, %N
IVCount: (-1 + (-1 * %N) + %M)
The condition on the backedge gets replaced with an "icmp ne", and the final assembly for the loop body is:
.LBB0_2:
addl (%esi), %eax
addl %edx, %esi
decl %ecx
jne .LBB0_2
With r225034, IndVar Simplify doesn't fire, we drag the "icmp slt" until the end, and end up with:
.LBB0_2:
addl (%esi), %eax
incl %edi
addl %edx, %esi
cmpl %ecx, %edi
jl .LBB0_2
Note that this is not the only issue we're seeing, some of the regressions don't appear to be IndVar-related.
It may be a good idea to revert r225034 and do more extensive performance testing before re-committing.
Thanks,
Michael
-----Original Message-----
From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of David Majnemer
Sent: Wednesday, December 31, 2014 06:22
To: llvm-commits at cs.uiuc.edu
Subject: [llvm] r225034 - InstCombine: try to transform A-B < 0 into A < B
Author: majnemer
Date: Tue Dec 30 22:21:41 2014
New Revision: 225034
URL: http://llvm.org/viewvc/llvm-project?rev=225034&view=rev
Log:
InstCombine: try to transform A-B < 0 into A < B
We are allowed to move the 'B' to the right hand side if we an prove there is no signed overflow and if the comparison itself is signed.
Modified:
llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
llvm/trunk/test/Transforms/InstCombine/icmp.ll
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=225034&r1=225033&r2=225034&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Tue
+++ Dec 30 22:21:41 2014
@@ -2692,6 +2692,26 @@ Instruction *InstCombiner::visitICmpInst
return new ICmpInst(I.getPredicate(), A, B);
}
+ // (icmp sgt (sub nsw A B), -1) -> (icmp sge A, B)
+ if (I.getPredicate() == ICmpInst::ICMP_SGT && CI->isAllOnesValue() &&
+ match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+ return new ICmpInst(ICmpInst::ICMP_SGE, A, B);
+
+ // (icmp sgt (sub nsw A B), 0) -> (icmp sgt A, B)
+ if (I.getPredicate() == ICmpInst::ICMP_SGT && CI->isZero() &&
+ match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+ return new ICmpInst(ICmpInst::ICMP_SGT, A, B);
+
+ // (icmp slt (sub nsw A B), 0) -> (icmp slt A, B)
+ if (I.getPredicate() == ICmpInst::ICMP_SLT && CI->isZero() &&
+ match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+ return new ICmpInst(ICmpInst::ICMP_SLT, A, B);
+
+ // (icmp slt (sub nsw A B), 1) -> (icmp sle A, B)
+ if (I.getPredicate() == ICmpInst::ICMP_SLT && CI->isOne() &&
+ match(Op0, m_NSWSub(m_Value(A), m_Value(B))))
+ return new ICmpInst(ICmpInst::ICMP_SLE, A, B);
+
// If we have an icmp le or icmp ge instruction, turn it into the
// appropriate icmp lt or icmp gt instruction. This allows us to rely on
// them being folded in the code below. The SimplifyICmpInst code has
Modified: llvm/trunk/test/Transforms/InstCombine/icmp.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/icmp.ll?rev=225034&r1=225033&r2=225034&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/icmp.ll Tue Dec 30 22:21:41
+++ 2014
@@ -1522,3 +1522,39 @@ define zeroext i1 @icmp_cmpxchg_strong(i
%icmp = icmp eq i32 %xtrc, %old_val
ret i1 %icmp
}
+
+; CHECK-LABEL: @f1
+; CHECK-NEXT: %[[cmp:.*]] = icmp sge i64 %a, %b ; CHECK-NEXT: ret i1
+%[[cmp]] define i1 @f1(i64 %a, i64 %b) {
+ %t = sub nsw i64 %a, %b
+ %v = icmp sge i64 %t, 0
+ ret i1 %v
+}
+
+; CHECK-LABEL: @f2
+; CHECK-NEXT: %[[cmp:.*]] = icmp sgt i64 %a, %b ; CHECK-NEXT: ret i1
+%[[cmp]] define i1 @f2(i64 %a, i64 %b) {
+ %t = sub nsw i64 %a, %b
+ %v = icmp sgt i64 %t, 0
+ ret i1 %v
+}
+
+; CHECK-LABEL: @f3
+; CHECK-NEXT: %[[cmp:.*]] = icmp slt i64 %a, %b ; CHECK-NEXT: ret i1
+%[[cmp]] define i1 @f3(i64 %a, i64 %b) {
+ %t = sub nsw i64 %a, %b
+ %v = icmp slt i64 %t, 0
+ ret i1 %v
+}
+
+; CHECK-LABEL: @f4
+; CHECK-NEXT: %[[cmp:.*]] = icmp sle i64 %a, %b ; CHECK-NEXT: ret i1
+%[[cmp]] define i1 @f4(i64 %a, i64 %b) {
+ %t = sub nsw i64 %a, %b
+ %v = icmp sle i64 %t, 0
+ ret i1 %v
+}
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