[llvm-dev] persuading licm to do the right thing

[Preston Briggs via llvm-dev]

When I compile two different modules using

clang -O -S -emit-llvm


I get different .ll files, no surprise.

The first looks like

double *v;

double zap(long n) {
  double sum = 0;
  for (long i = 0; i < n; i++)
    sum += v[i];
  return sum;
}


yielding

@v = common global double* null, align 8

; Function Attrs: nounwind readonly uwtable
define double @zap(i64 %n) #0 {
entry:
  %cmp4 = icmp sgt i64 %n, 0
  br i1 %cmp4, label %for.body.lr.ph, label %for.end

for.body.lr.ph:                                   ; preds = %entry
  %0 = load double** @v, align 8, !tbaa !1
  br label %for.body

for.body:                                         ; preds = %for.body, %
for.body.lr.ph
  %i.06 = phi i64 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
  %sum.05 = phi double [ 0.000000e+00, %for.body.lr.ph ], [ %add, %for.body
]
  %arrayidx = getelementptr inbounds double* %0, i64 %i.06
  %1 = load double* %arrayidx, align 8, !tbaa !5
  %add = fadd double %sum.05, %1
  %inc = add nsw i64 %i.06, 1

%exitcond = icmp eq i64 %inc, %n
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body,
%entry
  %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body ]
  ret double %sum.0.lcssa
}


and the second looks like

double v[10000];

double zap(long n) {
  double sum = 0;
  for (long i = 0; i < n; i++)
    sum += v[i];
  return sum;
}


yielding

; ModuleID = 'z.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-f128:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@v = common global [10000 x double] zeroinitializer, align 16

; Function Attrs: nounwind readonly uwtable
define double @zap(i64 %n) #0 {
entry:
  %cmp4 = icmp sgt i64 %n, 0
  br i1 %cmp4, label %for.body, label %for.end

for.body:                                         ; preds = %entry,
%for.body
  %i.06 = phi i64 [ %inc, %for.body ], [ 0, %entry ]
  %sum.05 = phi double [ %add, %for.body ], [ 0.000000e+00, %entry ]
  %arrayidx = getelementptr inbounds [10000 x double]* @v, i64 0, i64 %i.06
  %0 = load double* %arrayidx, align 8, !tbaa !1
  %add = fadd double %sum.05, %0
  %inc = add nsw i64 %i.06, 1
  %exitcond = icmp eq i64 %inc, %n
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body,
%entry
  %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body ]
  ret double %sum.0.lcssa
}

attributes #0 = { nounwind readonly uwtable "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
"unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"Clang Front-End version 3.4.1
(tags/RELEASE_34/final)"}
!1 = metadata !{metadata !2, metadata !2, i64 0}
!2 = metadata !{metadata !"double", metadata !3, i64 0}
!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
!4 = metadata !{metadata !"Simple C/C++ TBAA"}


(I included all the metadata and such for the 2nd case, on the off chance
it matters.)

Is there any way I can convince licm (or something) to rip open the GEP and
hoist the reference to @v outside the loop, similar to the first example?

Thanks,
Preston
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