[ELF] Improve OVERLAY tests

Also test two issues:

* When the start address is `.`, subsequent sections don't share the
  address of the first overlay section.
* When the first overlay section is empty and discardable, `p_paddr` is
  incorrectly zero. This is because a discarded section has a zero
  address, causing `prev->getLMA() + prev->size` where `prev` refers to
  the first section to evaluate to zero.

Author: MaskRay

lld/test/ELF/linkerscript/overlay-reject.test

@@ -1,31 +1,91 @@
 # REQUIRES: x86
-# RUN: echo 'nop; .section .small, "a"; .long 0; .section .big, "a"; .quad 1;' \
-# RUN:   | llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t.o
-# RUN: ld.lld %t.o --script %s -o %t
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
+# RUN: ld.lld a.o -T a.t -o a
 
-SECTIONS {
-  OVERLAY 0x1000 : AT ( 0x4000 ) {
-    .out.big { *(.big) } 
-    .out.small { *(.small) } 
-  } 
-}
-
-## Here we check that can handle OVERLAY which will produce sections 
+## Here we check that can handle OVERLAY which will produce sections
 ## .out.big and .out.small with the same starting VAs, but different LMAs.
 ## Section .big is larger than .small, we check that placing of section
 ## .text does not cause overlapping error and that
 ## .text's VA is 0x1000 + max(sizeof(.out.big), sizeof(.out.small)).
 
-# RUN: llvm-readelf --sections -l %t | FileCheck %s
+# RUN: llvm-readelf --sections -l a | FileCheck %s
 
-# CHECK: Section Headers:
-# CHECK: Name       Type     Address          Off    Size
-# CHECK: .out.big   PROGBITS 0000000000001000 001000 000008
-# CHECK: .out.small PROGBITS 0000000000001000 002000 000004
-# CHECK: .text      PROGBITS 0000000000001008 002008 000001
+# CHECK:      Name       Type     Address          Off    Size
+# CHECK:      .big1      PROGBITS 0000000000001000 001000 000008
+# CHECK-NEXT: .small1    PROGBITS 0000000000001000 002000 000004
+# CHECK-NEXT: .small2    PROGBITS 0000000000001008 002008 000004
+# CHECK-NEXT: .big2      PROGBITS 0000000000001008 003008 000008
+# CHECK-NEXT: .small3    PROGBITS 0000000000001010 003010 000004
+# CHECK-NEXT: .big3      PROGBITS 0000000000001014 003014 000008
+# CHECK-NEXT: .text      PROGBITS 0000000000001024 003024 000001
 
 # CHECK:      Program Headers:
 # CHECK:      Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
-# CHECK-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000004000 0x000008 0x000008 R   0x1000
-# CHECK-NEXT: LOAD 0x002000 0x0000000000001000 0x0000000000004008 0x000004 0x000004 R   0x1000
-# CHECK-NEXT: LOAD 0x002008 0x0000000000001008 0x0000000000004010 0x000001 0x000001 R E 0x1000
+# CHECK-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000008 0x000008 R   0x1000
+# CHECK-NEXT: LOAD 0x002000 0x0000000000001000 0x0000000000001008 0x000004 0x000004 R   0x1000
+# CHECK-NEXT: LOAD 0x002008 0x0000000000001008 0x0000000000002008 0x000004 0x000004 R   0x1000
+# CHECK-NEXT: LOAD 0x003008 0x0000000000001008 0x000000000000200c 0x000008 0x000008 R   0x1000
+## FIXME Fix p_paddr when the first section in an overlay is empty and discarded.
+# CHECK-NEXT: LOAD 0x003010 0x0000000000001010 0x0000000000000000 0x000004 0x000004 R   0x1000
+# CHECK-NEXT: LOAD 0x003014 0x0000000000001014 0x0000000000000004 0x000008 0x000008 R   0x1000
+# CHECK-NEXT: LOAD 0x003024 0x0000000000001024 0x0000000000000014 0x000001 0x000001 R E 0x1000
+
+# RUN: not ld.lld a.o -T err1.t 2>&1 | FileCheck %s --check-prefix=ERR1 --match-full-lines --strict-whitespace
+#      ERR1:{{.*}}error: err1.t:3: { expected, but got 0x3000
+# ERR1-NEXT:>>>     .out.aaa 0x3000 : { *(.aaa) }
+# ERR1-NEXT:>>>              ^
+
+# RUN: not ld.lld a.o -T err2.t 2>&1 | FileCheck %s --check-prefix=ERR2 --match-full-lines --strict-whitespace
+#      ERR2:{{.*}}error: err2.t:{{.*}}: { expected, but got AX
+# ERR2-NEXT:>>>     .out.aaa { *(.aaa) } > AX AT>FLASH
+# ERR2-NEXT:>>>                            ^
+
+#--- a.s
+.globl _start
+_start:
+  nop
+
+.section .small1, "a"; .long 0
+.section .big1, "a"; .quad 1
+
+.section .small2, "a"; .long 0
+.section .big2, "a"; .quad 1
+
+.section .small3, "a"; .long 0
+.section .big3, "a"; .quad 1
+
+#--- a.t
+SECTIONS {
+  OVERLAY 0x1000 : AT( 0x1000 ) {
+    .big1 { *(.big1) }
+    .small1 { *(.small1) }
+  }
+  OVERLAY 0x1008 : AT (0x2008) {
+    .small2 { *(.small2) }
+    .big2 { *(.big2) }
+  }
+  OVERLAY . : AT (0x2014) {
+    .empty3 { *(.empty3) }
+    .small3 { *(.small3) }
+    .big3 { *(.big3) }
+  }
+  .text : { *(.text) }
+}
+
+#--- err1.t
+SECTIONS {
+  OVERLAY 0x1000 : AT ( 0x2000 ) {
+    .out.aaa 0x3000 : { *(.aaa) }
+  }
+}
+
+#--- err2.t
+MEMORY {
+  AX (ax)    : ORIGIN = 0x3000, LENGTH = 0x4000
+}
+SECTIONS {
+  OVERLAY 0x1000 : AT ( 0x2000 ) {
+    .out.aaa { *(.aaa) } > AX AT>FLASH
+  }
+}