Add the .fallthrough control statement

This is exclusive to 'nasm' and it allows the developer to explicitly
tell the assembler than a "fall through" condition is actually desired:
it's not a mistake.

This comes in two flavors. The first, without arguments, just makes this
explicit without much enforcement. The second allows you to pass an
argument which is the name of the function or label you are expecting to
fall through. The assembler will error out if the fall through address
is not the expected one, hence telling the programmer whenever the fall
through condition they thought in the past is no longer true (e.g. the
function has moved somewhere else in the code).

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

5 files changed, 54 insertions, 0 deletions

diff --git a/tests/bad_fallthrough.s b/tests/bad_fallthrough.s
new file mode 100644
index 0000000..ac17ade
--- /dev/null
+++ b/tests/bad_fallthrough.s
@@ -0,0 +1,27 @@
+.segment "HEADER"
+    .byte 'N', 'E', 'S', $1A
+    .byte $02, $01
+    .byte $00
+    .byte $00
+
+.segment "CODE"
+
+.proc foo
+    lda #0
+    .fallthrough bar
+.endproc
+
+.fallthrough bar
+.fallthrough other
+ lda #0
+
+.proc bar
+    lda #0
+    .fallthrough other
+.endproc
+
+.proc other
+    rts
+.endproc
+
+.fallthrough other
diff --git a/tests/expected/bad_fallthrough.txt b/tests/expected/bad_fallthrough.txt
new file mode 100644
index 0000000..f105880
--- /dev/null
+++ b/tests/expected/bad_fallthrough.txt
@@ -0,0 +1,4 @@
+error: statement does not fall through (bad_fallthrough.s: line 11)
+error: statement does not fall through (bad_fallthrough.s: line 14)
+error: statement does not fall through (bad_fallthrough.s: line 15)
+error: statement does not fall through (bad_fallthrough.s: line 27)
diff --git a/tests/expected/fallthrough.nes b/tests/expected/fallthrough.nes
new file mode 100644
index 0000000..976f8fc
--- /dev/null
+++ b/tests/expected/fallthrough.nes
diff --git a/tests/expected/fallthrough.txt b/tests/expected/fallthrough.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tests/expected/fallthrough.txt
diff --git a/tests/fallthrough.s b/tests/fallthrough.s
new file mode 100644
index 0000000..2340ee8
--- /dev/null
+++ b/tests/fallthrough.s
@@ -0,0 +1,23 @@
+.segment "HEADER"
+    .byte 'N', 'E', 'S', $1A
+    .byte $02, $01
+    .byte $00
+    .byte $00
+
+.segment "CODE"
+
+.proc foo
+    lda #0
+    .fallthrough bar
+.endproc
+
+.fallthrough bar
+
+.proc bar
+    lda #0
+    .fallthrough other
+.endproc
+
+.proc other
+    rts
+.endproc