forked from OSchip/llvm-project
91 lines
2.6 KiB
LLVM
91 lines
2.6 KiB
LLVM
; XFAIL: *
|
|
; RUN: opt -S -basicaa -newgvn < %s | FileCheck %s
|
|
|
|
@a = external constant i32
|
|
; We can value forward across the fence since we can (semantically)
|
|
; reorder the following load before the fence.
|
|
define i32 @test(i32* %addr.i) {
|
|
; CHECK-LABEL: @test
|
|
; CHECK: store
|
|
; CHECK: fence
|
|
; CHECK-NOT: load
|
|
; CHECK: ret
|
|
store i32 5, i32* %addr.i, align 4
|
|
fence release
|
|
%a = load i32, i32* %addr.i, align 4
|
|
ret i32 %a
|
|
}
|
|
|
|
; Same as above
|
|
define i32 @test2(i32* %addr.i) {
|
|
; CHECK-LABEL: @test2
|
|
; CHECK-NEXT: fence
|
|
; CHECK-NOT: load
|
|
; CHECK: ret
|
|
%a = load i32, i32* %addr.i, align 4
|
|
fence release
|
|
%a2 = load i32, i32* %addr.i, align 4
|
|
%res = sub i32 %a, %a2
|
|
ret i32 %res
|
|
}
|
|
|
|
; We can not value forward across an acquire barrier since we might
|
|
; be syncronizing with another thread storing to the same variable
|
|
; followed by a release fence. This is not so much enforcing an
|
|
; ordering property (though it is that too), but a liveness
|
|
; property. We expect to eventually see the value of store by
|
|
; another thread when spinning on that location.
|
|
define i32 @test3(i32* noalias %addr.i, i32* noalias %otheraddr) {
|
|
; CHECK-LABEL: @test3
|
|
; CHECK: load
|
|
; CHECK: fence
|
|
; CHECK: load
|
|
; CHECK: ret i32 %res
|
|
; the following code is intented to model the unrolling of
|
|
; two iterations in a spin loop of the form:
|
|
; do { fence acquire: tmp = *%addr.i; ) while (!tmp);
|
|
; It's hopefully clear that allowing PRE to turn this into:
|
|
; if (!*%addr.i) while(true) {} would be unfortunate
|
|
fence acquire
|
|
%a = load i32, i32* %addr.i, align 4
|
|
fence acquire
|
|
%a2 = load i32, i32* %addr.i, align 4
|
|
%res = sub i32 %a, %a2
|
|
ret i32 %res
|
|
}
|
|
|
|
; We can forward the value forward the load
|
|
; across both the fences, because the load is from
|
|
; a constant memory location.
|
|
define i32 @test4(i32* %addr) {
|
|
; CHECK-LABEL: @test4
|
|
; CHECK-NOT: load
|
|
; CHECK: fence release
|
|
; CHECK: store
|
|
; CHECK: fence seq_cst
|
|
; CHECK: ret i32 0
|
|
%var = load i32, i32* @a
|
|
fence release
|
|
store i32 42, i32* %addr, align 8
|
|
fence seq_cst
|
|
%var2 = load i32, i32* @a
|
|
%var3 = sub i32 %var, %var2
|
|
ret i32 %var3
|
|
}
|
|
|
|
; Another example of why forwarding across an acquire fence is problematic
|
|
; can be seen in a normal locking operation. Say we had:
|
|
; *p = 5; unlock(l); lock(l); use(p);
|
|
; forwarding the store to p would be invalid. A reasonable implementation
|
|
; of unlock and lock might be:
|
|
; unlock() { atomicrmw sub %l, 1 unordered; fence release }
|
|
; lock() {
|
|
; do {
|
|
; %res = cmpxchg %p, 0, 1, monotonic monotonic
|
|
; } while(!%res.success)
|
|
; fence acquire;
|
|
; }
|
|
; Given we chose to forward across the release fence, we clearly can't forward
|
|
; across the acquire fence as well.
|
|
|