[LLVMbugs] [Bug 21914] New: [InstCombine] prematurely promoting floating point negation
bugzilla-daemon at llvm.org
bugzilla-daemon at llvm.org
Sun Dec 14 22:28:43 PST 2014
http://llvm.org/bugs/show_bug.cgi?id=21914
Bug ID: 21914
Summary: [InstCombine] prematurely promoting floating point
negation
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedbugs at nondot.org
Reporter: wujingyue at gmail.com
CC: llvmbugs at cs.uiuc.edu
Classification: Unclassified
The optimization that transforms (-x * y) to -(x * y) for floating pointer
numbers seems premature.
http://llvm.org/doxygen/InstCombineMulDivRem_8cpp_source.html#l00602
00602 if (Opnd0->hasOneUse()) {
00603 // -X * Y => -(X*Y) (Promote negation as high as possible)
00604 Value *T = Builder->CreateFMul(N0, Opnd1);
00605 Value *Neg = Builder->CreateFNeg(T);
00606 Neg->takeName(&I);
00607 return ReplaceInstUsesWith(I, Neg);
00608 }
While I understand the motivation of this transformation, it prevents potential
LICM if X is a loop invariant and Y is a loop variant. For instance,
int bar(int, int);
void foo(int n, float *output, float beta) {
float sum = 0;
for (int i = 0; i < n; ++i)
sum += bar(i, i * (-beta));
*output = sum;
}
With i*(-beta) transformed to -(i*beta), the LICM passes after instcombine is
unable to hoist -beta outside the loop. As a result, "opt -O3" leaves a fsub
instruction inside the loop:
define void @_Z3fooiPff(i32 %n, float* nocapture %output, float %beta) #0 {
entry:
%cmp7 = icmp sgt i32 %n, 0
br i1 %cmp7, label %for.body.lr.ph, label %for.end
for.body.lr.ph: ; preds = %entry
%0 = add i32 %n, -1
br label %for.body
for.body: ; preds = %for.body,
%for.body.lr.ph
%i.09 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
%sum.08 = phi float [ 0.000000e+00, %for.body.lr.ph ], [ %add, %for.body ]
%conv = sitofp i32 %i.09 to float
%1 = fmul float %conv, %beta
%mul = fsub float -0.000000e+00, %1
%conv1 = fptosi float %mul to i32
%call = tail call i32 @_Z3barii(i32 %i.09, i32 %conv1)
%conv2 = sitofp i32 %call to float
%add = fadd float %sum.08, %conv2
%inc = add nsw i32 %i.09, 1
%exitcond = icmp eq i32 %i.09, %0
br i1 %exitcond, label %for.end.loopexit, label %for.body
for.end.loopexit: ; preds = %for.body
%add.lcssa = phi float [ %add, %for.body ]
br label %for.end
for.end: ; preds = %for.end.loopexit,
%entry
%sum.0.lcssa = phi float [ 0.000000e+00, %entry ], [ %add.lcssa,
%for.end.loopexit ]
store float %sum.0.lcssa, float* %output, align 4, !tbaa !1
ret void
}
Jingyue
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