https://bugs.llvm.org/show_bug.cgi?id=52396
Bug ID: 52396
Summary: Folding common switch code
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
LLVM optimizes common switch cases but clang is not producing the best codegen
for the following example:
void foo(int x) {
switch(x) {
case 2: func(x); break;
case 3: func(x); break;
case 4: func(x); break;
case 5: func(x); break;
case 6: func(x); break;
default: __builtin_unreachable();
}
}
void bar(int x) {
switch(x) {
case 1: func(x); break;
case 4: func(x); break;
case 7: func(x); break;
case 3: func(x); break;
case 9: func(x); break;
default: __builtin_unreachable();
}
}
foo(int): # @foo(int)
jmp func2(int) # TAILCALL
bar(int): # @bar(int)
add edi, -1
movsxd rax, edi
mov edi, dword ptr [4*rax + .Lswitch.table.bar(int)]
jmp func2(int) # TAILCALL
.Lswitch.table.bar(int):
.long 1 # 0x1
.long 1 # 0x1
.long 3 # 0x3
.long 4 # 0x4
.long 1 # 0x1
.long 1 # 0x1
.long 7 # 0x7
.long 1 # 0x1
.long 9 # 0x9
https://godbolt.org/z/Ya34bneoT
It appears LLVM propagates the constant x early in the optimization, pulls out
the common calls to func(), then later eliminates the switches during
SimplifyCFG. LLVM optimizes foo based off the switch case values being
sequential, with non-sequential values it doesn't undo the constant
propagation:
*** IR Dump After InstCombinePass on _Z3bari ***
; Function Attrs: mustprogress uwtable
define dso_local void @_Z3bari(i32 %0) local_unnamed_addr #0 {
switch i32 %0, label %6 [
i32 1, label %7
i32 4, label %2
i32 7, label %3
i32 3, label %4
i32 9, label %5
]
2: ; preds = %1
br label %7
3: ; preds = %1
br label %7
4: ; preds = %1
br label %7
5: ; preds = %1
br label %7
6: ; preds = %1
unreachable
7: ; preds = %1, %5, %4, %3, %2
%8 = phi i32 [ 9, %5 ], [ 3, %4 ], [ 7, %3 ], [ 4, %2 ], [ 1, %1 ]
tail call void @_Z4funci(i32 %8)
ret void
}
*** IR Dump After SimplifyCFGPass on _Z3bari ***
; Function Attrs: mustprogress uwtable
define dso_local void @_Z3bari(i32 %0) local_unnamed_addr #0 {
%2 = sub i32 %0, 1
%3 = trunc i32 %2 to i16
%4 = lshr i16 333, %3
%5 = trunc i16 %4 to i1
call void @llvm.assume(i1 %5)
%6 = getelementptr inbounds [9 x i32], [9 x i32]* @switch.table._Z3bari, i32
0, i32 %2
%7 = load i32, i32* %6, align 4
tail call void @_Z4funci(i32 %7)
ret void
}
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