[Bug tree-optimization/26944] New: -ftree-ch generates worse code
dann at godzilla dot ics dot uci dot edu
gcc-bugzilla@gcc.gnu.org
Thu Mar 30 16:15:00 GMT 2006
The loop from the code below is compiled to this when using gcc-4.1 -O2
.L5:
movl 16(%ebp), %eax
addl %ecx, %eax
addl $1, %ecx
movl %edx, 20(%ebx,%eax,4)
leal (%edx,%ecx), %eax
cmpl %edi, %eax
jle .L5
but the code is much better when using gcc -fno-tree-ch -O2
.L3:
addl $1, %ecx
movl %ebx, -4(%edx)
addl $4, %edx
cmpl %eax, %ecx
jle .L3
This is a regression as gcc-3.4.3 generates similar code.
The code is from the Dhrystone as included in Unixbench.
The regression is quite important as embedded processor people still use
Dhrystone for benchmarking compiler/processor speed.
Its strange that tree-ch messes up, the loop is about as simple as loops can
get.
typedef int One_Fifty;
typedef int Arr_1_Dim [50];
typedef int Arr_2_Dim [50] [50];
extern int Int_Glob;
void Proc_8 (Arr_1_Par_Ref, Arr_2_Par_Ref, Int_1_Par_Val, Int_2_Par_Val)
Arr_1_Dim Arr_1_Par_Ref;
Arr_2_Dim Arr_2_Par_Ref;
int Int_1_Par_Val;
int Int_2_Par_Val;
{
register One_Fifty Int_Index;
register One_Fifty Int_Loc;
Int_Loc = Int_1_Par_Val + 5;
Arr_1_Par_Ref [Int_Loc] = Int_2_Par_Val;
Arr_1_Par_Ref [Int_Loc+1] = Arr_1_Par_Ref [Int_Loc];
Arr_1_Par_Ref [Int_Loc+30] = Int_Loc;
for (Int_Index = Int_Loc; Int_Index <= Int_Loc+1; ++Int_Index)
Arr_2_Par_Ref [Int_Loc] [Int_Index] = Int_Loc;
Arr_2_Par_Ref [Int_Loc] [Int_Loc-1] += 1;
Arr_2_Par_Ref [Int_Loc+20] [Int_Loc] = Arr_1_Par_Ref [Int_Loc];
Int_Glob = 5;
}
Intel's compiler generates even tighter code:
..B1.7: # Preds ..B1.10 ..B1.7
movl %ebx, (%ecx,%edx,4) #20.5
addl $1, %edx #19.55
cmpl %eax, %edx #19.3
jle ..B1.7 # Prob 80% #19.3
--
Summary: -ftree-ch generates worse code
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dann at godzilla dot ics dot uci dot edu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26944
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