Hi folks!
In /lib/Transforms/Utils/SimplifyCFG.cpp is optimization code which converts a switch statement to a table lookup but has problems when there are holes in the cases list and the default case can not be served with the table.
My first attempt to fix this is done by additionally testing a small set.
As an example the function
==>
unsigned test(unsigned x) {
switch (x) {
case 100: return 0;
case 101: return 1;
case 103: return 2;
case 105: return 3;
case 107: return 4;
case 109: return 5;
case 110: return 6;
default: return x*3;
}
}
<==
can be converted to
==>
.file "t.cpp"
.text
.globl _Z4testj
.align 16, 0x90
.type _Z4testj,@function
_Z4testj: # @_Z4testj
.cfi_startproc
# BB#0: # %entry
# kill: EDI<def> EDI<kill> RDI<def>
leal -100(%rdi), %eax
cmpl $11, %eax
jae .LBB0_3
# BB#1: # %switch.lookup
movl $1707, %ecx # imm = 0x6AB
btl %eax, %ecx
jae .LBB0_3
# BB#2: # %switch.lookup2
cltq
movl .Lswitch.table(,%rax,4), %eax
retq
.LBB0_3: # %sw.default
leal (%rdi,%rdi,2), %eax
retq
.Ltmp0:
.size _Z4testj, .Ltmp0-_Z4testj
.cfi_endproc
.type .Lswitch.table,@object # @switch.table
.section .rodata,"a",@progbits
.align 16
.Lswitch.table:
.long 0 # 0x0
.long 1 # 0x1
.long 0 # 0x0
.long 2 # 0x2
.long 0 # 0x0
.long 3 # 0x3
.long 0 # 0x0
.long 4 # 0x4
.long 0 # 0x0
.long 5 # 0x5
.long 6 # 0x6
.size .Lswitch.table, 44
.ident "clang version 3.5 (trunk 200617)"
.section ".note.GNU-stack","",@progbits
<==
By the way, what for is "cltq" just before accessing the value table?
The upper dword of rax should be zero.
Unfortunately my patch does not always work and the compiler crashes.
What have I overlooked? Please help.
Best regards
Jasper
PS: I posted a very similar message to llvm-commits@cs.uiuc.edu but got no response; perhaps this was the wrong place to discuss it.
patch.txt (3.89 KB)