[PATCH] D44911: [MachineCopyPropagation] Handle COPY with overlapping source/dest.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
efriedma added reviewers: gberry, qcolombet, MatzeB.
Herald added subscribers: kristof.beyls, javed.absar.

MachineCopyPropagation::CopyPropagateBlock has a bunch of special handling for COPY instructions.  This handling assumes that COPY instructions do not modify the source of the copy; this is wrong if the COPY destination overlaps the source.

To fix the bug, check explicitly for this situation, and fall back to the generic instruction handling.

This bug can't happen for most register classes because they don't have this sort of overlap, but there are a few register classes where this is possible.  The testcase uses the AArch64 QQQQ register class.


Repository:
  rL LLVM

https://reviews.llvm.org/D44911

Files:
  lib/CodeGen/MachineCopyPropagation.cpp
  test/CodeGen/AArch64/machine-dead-copy.mir


Index: test/CodeGen/AArch64/machine-dead-copy.mir
===================================================================
--- test/CodeGen/AArch64/machine-dead-copy.mir
+++ test/CodeGen/AArch64/machine-dead-copy.mir
@@ -6,6 +6,8 @@
   define i32 @copyprop2(i32 %a, i32 %b) { ret i32 %a }
   define i32 @copyprop3(i32 %a, i32 %b) { ret i32 %a }
   define i32 @copyprop4(i32 %a, i32 %b) { ret i32 %a }
+  define i32 @copyprop5(i32 %a, i32 %b) { ret i32 %a }
+  define i32 @copyprop6(i32 %a, i32 %b) { ret i32 %a }
   declare i32 @foo(i32)
 ...
 ---
@@ -65,3 +67,33 @@
     RET_ReallyLR implicit $w0
 ...
 
+# Don't try to erase any COPY which overlaps itself.
+# CHECK-LABEL: name: copyprop5
+# CHECK: bb.0:
+# CHECK: COPY killed $q26_q27_q28_q29
+# CHECK: COPY killed $q28_q29_q30_q31
+name: copyprop5
+body: |
+  bb.0:
+    liveins: $q26_q27_q28_q29
+    $q28_q29_q30_q31 = COPY killed $q26_q27_q28_q29
+    $q26_q27_q28_q29 = COPY killed $q28_q29_q30_q31
+    BL @foo, csr_aarch64_aapcs, implicit killed $q26_q27_q28_q29
+    RET_ReallyLR
+...
+
+# Don't try to analyze any COPY which overlaps itself.
+# CHECK-LABEL: name: copyprop6
+# CHECK: bb.0:
+# CHECK: COPY killed $q26_q27_q28_q29
+# CHECK: $q30 = COPY $q28
+name: copyprop6
+body: |
+  bb.0:
+    liveins: $q26_q27_q28_q29
+    $q28_q29_q30_q31 = COPY killed $q26_q27_q28_q29
+    $q30 = COPY $q28
+    BL @foo, csr_aarch64_aapcs, implicit killed $q28_q29_q30_q31
+    RET_ReallyLR
+...
+
Index: lib/CodeGen/MachineCopyPropagation.cpp
===================================================================
--- lib/CodeGen/MachineCopyPropagation.cpp
+++ lib/CodeGen/MachineCopyPropagation.cpp
@@ -399,7 +399,9 @@
     MachineInstr *MI = &*I;
     ++I;
 
-    if (MI->isCopy()) {
+    // Analyze copies (which don't overlap themselves).
+    if (MI->isCopy() && !TRI->regsOverlap(MI->getOperand(0).getReg(),
+                                          MI->getOperand(1).getReg())) {
       unsigned Def = MI->getOperand(0).getReg();
       unsigned Src = MI->getOperand(1).getReg();
 


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