| Issue |
165696
|
| Summary |
[BPF] __bpf_trap declaration emitted even if not called
|
| Labels |
backend:BPF
|
| Assignees |
|
| Reporter |
nikic
|
For this IR:
```llvm
target triple = "bpf"
define i32 @test(i8 %x) {
entry:
%0 = and i8 %x, 3
switch i8 %0, label %default.unreachable4 [
i8 0, label %return
i8 1, label %sw.bb1
i8 2, label %sw.bb2
i8 3, label %sw.bb3
]
sw.bb1: ; preds = %entry
br label %return
sw.bb2: ; preds = %entry
br label %return
sw.bb3: ; preds = %entry
br label %return
default.unreachable4: ; preds = %entry
unreachable
return: ; preds = %entry, %sw.bb3, %sw.bb2, %sw.bb1
%retval.0 = phi i32 [ 12, %sw.bb1 ], [ 43, %sw.bb2 ], [ 54, %sw.bb3 ], [ 32, %entry ]
ret i32 %retval.0
}
```
llc produces:
```
.file "<stdin>"
.text
.globl test # -- Begin function test
.p2align 3
.type test,@function
test: # @test
.cfi_startproc
# %bb.0: # %entry
w1 &= 3
if w1 s> 1 goto LBB0_3
# %bb.1: # %entry
w0 = 32
if w1 == 0 goto LBB0_6
# %bb.2: # %entry
w0 = 12
goto LBB0_6
LBB0_3: # %entry
if w1 == 2 goto LBB0_5
# %bb.4: # %entry
w0 = 54
goto LBB0_6
LBB0_5: # %sw.bb2
w0 = 43
LBB0_6: # %return
exit
.Lfunc_end0:
.size test, .Lfunc_end0-test
.cfi_endproc
# -- End function
.weak __bpf_trap
```
Note that it declares `.weak __bpf_trap`, even though the function is never called. This results in a `libbpf: extern weak function __bpf_trap is unsupported` error on kernels that don't have support for `__bpf_trap`. On kernels that do have it, there is no error because there is no call.
The reason is that the `default.unreachable4` gets lowered to a call to `__bpf_trap` originally and then gets eliminated as an unused block.
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