https://llvm.org/bugs/show_bug.cgi?id=27871
Bug ID: 27871
Summary: Problematic loop-carried variable widening by
InstCombine
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: m...@manueljacob.de
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Given the following C code:
int test(size_t n, char *a) {
char sum = 0;
for (size_t i = 0; i < n; ++i)
sum += a[i];
return sum;
}
After running a few simplifications (SROA, SimplifyCFG, LoopRotate) the IR
looks like this:
define i32 @test(i64 %n, i8* %a) #0 {
entry:
%cmp4 = icmp ult i64 0, %n
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 i8 [ %conv2, %for.body ], [ 0, %entry ]
%arrayidx = getelementptr inbounds i8, i8* %a, i64 %i.06
%0 = load i8, i8* %arrayidx, align 1
%conv = sext i8 %0 to i32
%conv1 = sext i8 %sum.05 to i32
%add = add nsw i32 %conv1, %conv
%conv2 = trunc i32 %add to i8
%inc = add i64 %i.06, 1
%cmp = icmp ult i64 %inc, %n
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %for.body, %entry
%sum.0.lcssa = phi i8 [ 0, %entry ], [ %conv2, %for.body ]
%conv3 = sext i8 %sum.0.lcssa to i32
ret i32 %conv3
}
I expected that running InstCombine produces code similar to this (transforming
the sext, sext, add, trunc chain to a single add instruction):
define i32 @test(i64 %n, i8* %a) #0 {
entry:
%cmp4 = icmp ult i64 0, %n
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 i8 [ %add, %for.body ], [ 0, %entry ]
%arrayidx = getelementptr inbounds i8, i8* %a, i64 %i.06
%0 = load i8, i8* %arrayidx, align 1
%add = add nsw i8 %sum.05, %0
%inc = add i64 %i.06, 1
%cmp = icmp ult i64 %inc, %n
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %for.body, %entry
%sum.0.lcssa = phi i8 [ 0, %entry ], [ %add, %for.body ]
%conv3 = sext i8 %sum.0.lcssa to i32
ret i32 %conv3
}
However instead InstCombine widened %sum.05 to i32 type, adding several shifts
to fix up the width:
define i32 @test(i64 %n, i8* %a) #0 {
entry:
%cmp4 = icmp eq i64 %n, 0
br i1 %cmp4, label %for.end, label %for.body
for.body: ; preds = %entry, %for.body
%i.06 = phi i64 [ %inc, %for.body ], [ 0, %entry ]
%sum.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]
%arrayidx = getelementptr inbounds i8, i8* %a, i64 %i.06
%0 = load i8, i8* %arrayidx, align 1
%conv = sext i8 %0 to i32
%sext7 = shl i32 %sum.05, 24
%conv1 = ashr exact i32 %sext7, 24
%add = add nsw i32 %conv1, %conv
%inc = add i64 %i.06, 1
%cmp = icmp ult i64 %inc, %n
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %entry, %for.body
%sum.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
%sext = shl i32 %sum.0.lcssa, 24
%conv3 = ashr exact i32 %sext, 24
ret i32 %conv3
}
Running opt -O3 again on the IR doesn't improve the code. Instead it generates
very awful code. I've attached it for reference, but there seems to be an
unrelated problem involved.
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