http://llvm.org/bugs/show_bug.cgi?id=14241
Bug #: 14241
Summary: LoopIdiomRecognize forms an invalid memcpy
Product: new-bugs
Version: unspecified
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected], [email protected],
[email protected]
Classification: Unclassified
Consider the following test case:
% cat bug.cpp
struct S { int v; };
void f(S *s, unsigned size) {
S *i = s, *e = s + size - 1;
while (i != e) {
*i = *(i + 1);
++i;
}
}
% ./bin/clang -S -emit-llvm -o - -O1 bug.cpp
; ModuleID = 'bug.cpp'
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-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
%struct.S = type { i32 }
define void @_Z1fP1Sj(%struct.S* %s, i32 %size) nounwind uwtable {
entry:
%cmp7 = icmp eq i32 %size, 1
br i1 %cmp7, label %while.end, label %while.cond.while.end_crit_edge
while.cond.while.end_crit_edge: ; preds = %entry
%s9 = bitcast %struct.S* %s to i8*
%scevgep = getelementptr %struct.S* %s, i64 1
%scevgep10 = bitcast %struct.S* %scevgep to i8*
%0 = zext i32 %size to i64
%1 = shl nuw nsw i64 %0, 2
%2 = add i64 %1, -4
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %s9, i8* %scevgep10, i64 %2, i32 4,
i1 false)
br label %while.end
while.end: ; preds =
%while.cond.while.end_crit_edge, %entry
ret void
}
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32,
i1) nounwind
Note that we produce a single memcpy call based on the loop trip count. This
isn't valid because the destination overlaps with the source. We need to form
memmove if we do this optimization at all.
Many thanks to David Blaikie for the excellent reduction here.
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