[SplitKit] Only copy live lanes

When splitting a live interval with subranges, only insert copies for
the lanes that are live at the point of the split. This avoids some
unnecessary copies and fixes a problem where copying dead lanes was
generating MIR that failed verification. The test case for this is
test/CodeGen/AMDGPU/splitkit-copy-live-lanes.mir.

Without this fix, some earlier live range splitting would create %430:

%430 [256r,848r:0)[848r,2584r:1)  0@256r 1@848r L0000000000000003 [848r,2584r:0)  0@848r L0000000000000030 [256r,2584r:0)  0@256r weight:1.480938e-03
...
256B     undef %430.sub2:vreg_128 = V_LSHRREV_B32_e32 16, %20.sub1:vreg_128, implicit $exec
...
848B     %430.sub0:vreg_128 = V_AND_B32_e32 %92:sreg_32, %20.sub1:vreg_128, implicit $exec
...
2584B    %431:vreg_128 = COPY %430:vreg_128

Then RAGreedy::tryLocalSplit would split %430 into %432 and %433 just
before 848B giving:

%432 [256r,844r:0)  0@256r L0000000000000030 [256r,844r:0)  0@256r weight:3.066802e-03
%433 [844r,848r:0)[848r,2584r:1)  0@844r 1@848r L0000000000000030 [844r,2584r:0)  0@844r L0000000000000003 [844r,844d:0)[848r,2584r:1)  0@844r 1@848r weight:2.831776e-03
...
256B     undef %432.sub2:vreg_128 = V_LSHRREV_B32_e32 16, %20.sub1:vreg_128, implicit $exec
...
844B     undef %433.sub0:vreg_128 = COPY %432.sub0:vreg_128 {
           internal %433.sub2:vreg_128 = COPY %432.sub2:vreg_128
848B     }
  %433.sub0:vreg_128 = V_AND_B32_e32 %92:sreg_32, %20.sub1:vreg_128, implicit $exec
...
2584B    %431:vreg_128 = COPY %433:vreg_128

Note that the copy from %432 to %433 at 844B is a curious
bundle-without-a-BUNDLE-instruction that SplitKit creates deliberately,
and it includes a copy of .sub0 which is not live at this point, and
that causes it to fail verification:

*** Bad machine code: No live subrange at use ***
- function:    zextload_global_v64i16_to_v64i64
- basic block: %bb.0  (0x7faed48) [0B;2848B)
- instruction: 844B    undef %433.sub0:vreg_128 = COPY %432.sub0:vreg_128
- operand 1:   %432.sub0:vreg_128
- interval:    %432 [256r,844r:0)  0@256r L0000000000000030 [256r,844r:0)  0@256r weight:3.066802e-03
- at:          844B

Using real bundles with a BUNDLE instruction might also fix this
problem, but the current fix is less invasive and also avoids some
unnecessary copies.

https://bugs.llvm.org/show_bug.cgi?id=47492

Differential Revision: https://reviews.llvm.org/D87757

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SplitKit.cpp b/llvm/lib/CodeGen/SplitKit.cpp
index 372c7f8..4029c85 100644
--- a/llvm/lib/CodeGen/SplitKit.cpp
+++ b/llvm/lib/CodeGen/SplitKit.cpp
@@ -649,10 +649,13 @@ VNInfo *SplitEditor::defFromParent(unsigned RegIdx,
   }
   if (!DidRemat) {
     LaneBitmask LaneMask;
-    if (LI->hasSubRanges()) {
+    if (OrigLI.hasSubRanges()) {
       LaneMask = LaneBitmask::getNone();
-      for (LiveInterval::SubRange &S : LI->subranges())
-        LaneMask |= S.LaneMask;
+      for (LiveInterval::SubRange &S : OrigLI.subranges()) {
+        if (S.liveAt(UseIdx))
+          LaneMask |= S.LaneMask;
+      }
+      assert(LaneMask.any() && "Interval has no live subranges");
     } else {
       LaneMask = LaneBitmask::getAll();
     }