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update this entry, now that Anton implemented shift/and lowering for

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switches.  There is one really easy isel thing here with tst we are not
getting.

llvm-svn: 37400
This commit is contained in:
Chris Lattner 2007-06-02 18:45:14 +00:00
parent f37bc8207e
commit 8a38eebb69
1 changed files with 66 additions and 17 deletions
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83
lib/Target/ARM/README-Thumb.txt
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@ -67,25 +67,9 @@ LPCRELL0:
//===---------------------------------------------------------------------===//
We compiles the following using a jump table.
We compiles the following:
define i16 @func_entry_2E_ce(i32 %i) {
newFuncRoot:
br label %entry.ce
bb12.exitStub: ; preds = %entry.ce
ret i16 0
bb4.exitStub: ; preds = %entry.ce, %entry.ce, %entry.ce
ret i16 1
bb9.exitStub: ; preds = %entry.ce, %entry.ce, %entry.ce
ret i16 2
bb.exitStub: ; preds = %entry.ce
ret i16 3
entry.ce: ; preds = %newFuncRoot
switch i32 %i, label %bb12.exitStub [
i32 0, label %bb4.exitStub
i32 1, label %bb9.exitStub
@ -95,8 +79,58 @@ entry.ce: ; preds = %newFuncRoot
i32 8, label %bb.exitStub
i32 9, label %bb9.exitStub
]
bb12.exitStub:
ret i16 0
bb4.exitStub:
ret i16 1
bb9.exitStub:
ret i16 2
bb.exitStub:
ret i16 3
}
into:
_func_entry_2E_ce:
mov r2, #1
lsl r2, r0
cmp r0, #9
bhi LBB1_4 @bb12.exitStub
LBB1_1: @newFuncRoot
mov r1, #13
tst r2, r1
bne LBB1_5 @bb4.exitStub
LBB1_2: @newFuncRoot
ldr r1, LCPI1_0
tst r2, r1
bne LBB1_6 @bb9.exitStub
LBB1_3: @newFuncRoot
mov r1, #1
lsl r1, r1, #8
tst r2, r1
bne LBB1_7 @bb.exitStub
LBB1_4: @bb12.exitStub
mov r0, #0
bx lr
LBB1_5: @bb4.exitStub
mov r0, #1
bx lr
LBB1_6: @bb9.exitStub
mov r0, #2
bx lr
LBB1_7: @bb.exitStub
mov r0, #3
bx lr
LBB1_8:
.align 2
LCPI1_0:
.long 642
gcc compiles to:
cmp r0, #9
@ -124,6 +158,21 @@ L12:
.align 2
L11:
.long 642
GCC is doing a couple of clever things here:
1. It is predicating one of the returns. This isn't a clear win though: in
cases where that return isn't taken, it is replacing one condbranch with
two 'ne' predicated instructions.
2. It is sinking the shift of "1 << i" into the tst, and using ands instead of
tst. This will probably require whole function isel.
3. GCC emits:
tst r1, #256
we emit:
mov r1, #1
lsl r1, r1, #8
tst r2, r1
//===---------------------------------------------------------------------===//