| Issue |
137277
|
| Summary |
Uniformity Analysis: incorrect control-divergence result.
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
jgu222
|
The uniform analysis fails to recognize PHI node as divergence.
The test (phi_issue.ll) using AMDGPU to get divergence branch. The uniformity
analysis marks all PHI nodes at B6 as uniform, which is incorrect.
Basically, it is the following simple if-then-else (except there are multiple
basic blocks in both then and else brances):
div-if ; divergent branch
/ \
x0=1 x1 = 2
\ /
\ /
x = phi x0, x1
x is marked as UNIFORM, which is incorrect as div-if is divergent branch
===============================================
;
; opt -mtriple amdgcn-unknown-amdhsa -passes='print<uniformity>' -disable-output phi_issue.ll
;
define amdgpu_kernel void @test_ctrl_divergence(i32 %a, i32 %b, i32 %c, i32 %d) {
Entry:
%tid = call i32 @llvm.amdgcn.workitem.id.x()
%div.cond = icmp eq i32 %tid, 0
br i1 %div.cond, label %B3, label %B0 ; divergent branch
B0:
%a0 = add i32 %a, 1
br label %B1
B1:
%b0 = add i32 %b, 2
br label %B2
B2:
%c0 = add i32 %c, 3
br label %B6
B3:
%a1 = add i32 %a, 10
br label %B4
B4:
%b1 = add i32 %b, 20
br label %B5
B5:
%c1 = add i32 %c, 30
br label %B6
B6:
%div_a = phi i32 [%a0, %B2], [%a1, %B5]
%div_b = phi i32 [%b0, %B2], [%b1, %B5]
%div_c = phi i32 [%c0, %B2], [%c1, %B5]
br i1 %div.cond, label %B8, label %B7 ; divergent branch
B7:
%d1 = add i32 %d, 1
br label %B8
B8:
%div_d = phi i32 [%d1, %B7], [%d, %B6]
ret void
}
declare i32 @llvm.amdgcn.workitem.id.x() #0
attributes #0 = {nounwind readnone }
====================================
%opt -mtriple amdgcn-unknown-amdhsa -passes='print<uniformity>' -disable-output phi_issue.ll
...
LOCK Entry
DEFINITIONS
DIVERGENT: %tid = call i32 @llvm.amdgcn.workitem.id.x()
DIVERGENT: %div.cond = icmp eq i32 %tid, 0
TERMINATORS
DIVERGENT: br i1 %div.cond, label %B3, label %B0
END BLOCK
......
BLOCK B6
DEFINITIONS
%div_a = phi i32 [ %a0, %B2 ], [ %a1, %B5 ]
%div_b = phi i32 [ %b0, %B2 ], [ %b1, %B5 ]
%div_c = phi i32 [ %c0, %B2 ], [ %c1, %B5 ]
TERMINATORS
DIVERGENT: br i1 %div.cond, label %B8, label %B7
END BLOCK
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