http://llvm.org/bugs/show_bug.cgi?id=16405
Bug ID: 16405
Summary: Missed scalar optimization
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
# cat 1.cc
double *f(double *p, int n) {
for (int i = 0; i < n; ++i)
++p;
return p;
}
# clang++ -S -emit-llvm -O2 1.cc -o -
define double* @_Z1fPdi(double* %p, i32 %n) #0 {
entry:
%cmp2 = icmp sgt i32 %n, 0
br i1 %cmp2, label %for.body.lr.ph, label %for.end
for.body.lr.ph: ; preds = %entry
%0 = add i32 %n, -1
%1 = zext i32 %0 to i64
%2 = add i64 %1, 1
%scevgep = getelementptr double* %p, i64 %2
br label %for.end
for.end: ; preds = %for.body.lr.ph,
%entry
%p.addr.0.lcssa = phi double* [ %scevgep, %for.body.lr.ph ], [ %p, %entry ]
ret double* %p.addr.0.lcssa
}
Looks like in for.body.lr.ph BB %n > 0. This allows replacing the first 3
instructions with one zext (or sext). This would also result in simpler X86
asm, too. Current code compiles to:
testl %esi, %esi
jle .LBB0_2
# BB#1: # %for.body.lr.ph
decl %esi
leaq 8(%rdi,%rsi,8), %rdi
.LBB0_2: # %for.end
movq %rdi, %rax
ret
--
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