From ec5e6cbabb3fad5c61dd7106eaaffe6fb1de6b29 Mon Sep 17 00:00:00 2001
From: Rangi <sylvie.oukaour+rangi42@gmail.com>
Date: Tue, 21 Jul 2026 14:51:58 -0400
Subject: [PATCH] Handle `-Wtruncation` for link-time `jr` values

Fixes #2028
---
 src/link/patch.cpp                  |   9 ++++++---
 test/link/jr-truncation/a.asm       |   2 ++
 test/link/jr-truncation/b.asm       |   1 +
 test/link/jr-truncation/out.err     |   2 ++
 test/link/jr-truncation/ref.out.bin | Bin 0 -> 16384 bytes
 test/link/jr-truncation/ref.out.sym |   2 ++
 test/link/test.sh                   |  11 +++++++++++
 7 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 test/link/jr-truncation/a.asm
 create mode 100644 test/link/jr-truncation/b.asm
 create mode 100644 test/link/jr-truncation/out.err
 create mode 100644 test/link/jr-truncation/ref.out.bin
 create mode 100644 test/link/jr-truncation/ref.out.sym

diff --git a/src/link/patch.cpp b/src/link/patch.cpp
index 8a0e43f7..4efca6a3 100644
--- a/src/link/patch.cpp
+++ b/src/link/patch.cpp
@@ -585,12 +585,15 @@ static void applyFilePatches(Section &section, Section &dataSection) {
 				rpnErrorAt(patch, "PC has no value outside of a section");
 				dataSection.data[offset] = 0;
 			} else {
+				// A `jr` is *encoded* in ROM as a 1-byte (8-bit) offset, so here `typeSize == 8`,
+				// but the object's *value* size is a 16-bit absolute address, so we pass 16 here.
+				checkPatchSize(patch, value, 16);
 				// Offset is relative to the byte *after* the operand
 				// PC as operand to `jr` is lower than reference PC by 2
 				uint16_t address = patch.pcSection->org + patch.pcOffset + 2;
-				// The truncation of `value - address` is intentional, since
-				// a low ROM0 address may `jr` to a high HRAM one.
-				int16_t jumpOffset = value - address;
+				// The 16-bit truncation of `value - address` is intentional, since
+				// a low ROM0 address may `jr` backwards to a high HRAM one.
+				int16_t jumpOffset = static_cast<int16_t>(value - address);
 
 				if (jumpOffset < -128 || jumpOffset > 127) {
 					firstErrorAt(
diff --git a/test/link/jr-truncation/a.asm b/test/link/jr-truncation/a.asm
new file mode 100644
index 00000000..59313abc
--- /dev/null
+++ b/test/link/jr-truncation/a.asm
@@ -0,0 +1,2 @@
+SECTION "jr", ROM0
+	jr CONSTANT
diff --git a/test/link/jr-truncation/b.asm b/test/link/jr-truncation/b.asm
new file mode 100644
index 00000000..be516d73
--- /dev/null
+++ b/test/link/jr-truncation/b.asm
@@ -0,0 +1 @@
+EXPORT DEF CONSTANT EQU $1234_0078
diff --git a/test/link/jr-truncation/out.err b/test/link/jr-truncation/out.err
new file mode 100644
index 00000000..4f2e1af8
--- /dev/null
+++ b/test/link/jr-truncation/out.err
@@ -0,0 +1,2 @@
+warning: Value $12340078 is not 16-bit [-Wtruncation]
+    at jr-truncation/a.asm(2)
diff --git a/test/link/jr-truncation/ref.out.bin b/test/link/jr-truncation/ref.out.bin
new file mode 100644
index 0000000000000000000000000000000000000000..2d59eb9333170054ddfbb44ddf4b6f0306ed2452
GIT binary patch
literal 16384
zcmeIuF#!Mo3<99VFzNCe!v`XeKMWW!V8DO@0|pEjFkrxd0RsjM7%*VKfB^#r3>YwA
tz<>b*1`HT5V8DO@0|pEjFkrxd0RsjM7%*VKfB^#r3>YwAz<`1AKmoy*0FM9w

literal 0
HcmV?d00001

diff --git a/test/link/jr-truncation/ref.out.sym b/test/link/jr-truncation/ref.out.sym
new file mode 100644
index 00000000..d05a90ab
--- /dev/null
+++ b/test/link/jr-truncation/ref.out.sym
@@ -0,0 +1,2 @@
+; File generated by rgblink
+12340078 CONSTANT
diff --git a/test/link/test.sh b/test/link/test.sh
index cb30efc3..f1f48656 100755
--- a/test/link/test.sh
+++ b/test/link/test.sh
@@ -201,6 +201,17 @@ tryCmpRom "$test"/ref.out.bin
 tryDiff "$test"/ref.out.sym "$outtemp2"
 evaluateTest
 
+test="jr-truncation"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$outtemp" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -n "$outtemp2" "$otemp" "$outtemp" 2>"$outtemp3"
+tryDiff "$test"/out.err "$outtemp3"
+tryCmpRom "$test"/ref.out.bin
+tryDiff "$test"/ref.out.sym "$outtemp2"
+evaluateTest
+
 test="high-low"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm