From 871ddc392df3e10be1a4b9dba9d0eaeaa37d1928 Mon Sep 17 00:00:00 2001
From: nella <simontupy64@gmail.com>
Date: Fri, 6 Nov 2026 16:07:00 +0100
Subject: [PATCH] Fix x87 int conversion.

---
 src/backend/pass/emit/x86/x86_lower_ops.cpp            |  9 +++++++--
 .../correctness/custom/x87_from_narrow_int_noopt.c     | 10 ++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)
 create mode 100644 src/compiler/test/correctness/custom/x87_from_narrow_int_noopt.c

diff --git a/src/backend/pass/emit/x86/x86_lower_ops.cpp b/src/backend/pass/emit/x86/x86_lower_ops.cpp
index c48f4b7..d8f56ba 100644
--- a/src/backend/pass/emit/x86/x86_lower_ops.cpp
+++ b/src/backend/pass/emit/x86/x86_lower_ops.cpp
@@ -1038,9 +1038,14 @@ namespace rat {
 		case Opcode::FPTrunc:
 			fstpSse(vregFor(n), opWidth(n->getType()), x);
 			return;
-		case Opcode::SIToFP:
-			fild(x87SlotOf(n), s);
+		case Opcode::SIToFP: {
+			// fild reads all 64 bits, a narrower source loaded from memory is zero extended
+			VReg wide = fresh(detail::kGp);
+			mov(wide, s);
+			signExtBits(wide, intBits(st));
+			fild(x87SlotOf(n), wide);
 			return;
+		}
 		case Opcode::UIToFP:
 			emitUIntToX87(n, s, intBits(st));
 			return;
diff --git a/src/compiler/test/correctness/custom/x87_from_narrow_int_noopt.c b/src/compiler/test/correctness/custom/x87_from_narrow_int_noopt.c
new file mode 100644
index 0000000..c4a08c1
--- /dev/null
+++ b/src/compiler/test/correctness/custom/x87_from_narrow_int_noopt.c
@@ -0,0 +1,10 @@
+// expect: 1
+// passes:
+
+// a negative int read back from memory, converted to long double
+long double widen(int s) {
+	int* p = &s;
+	return *p;
+}
+
+int main(void) { return widen(-1) == -1.0L; }