From 75c91feb3619836cd48d57ab7b00d626b5b53596 Mon Sep 17 00:00:00 2001
From: gingerBill <gingerBill@users.noreply.github.com>
Date: Wed, 19 Aug 2026 12:56:03 +0100
Subject: [PATCH] Check for output parameters not used but pinned; Check for
 templates with observable effects but not marked as `#volatile`

---
 src/check_asm.cpp | 25 ++++++++++++++++++++++++-
 src/entity.cpp    |  3 +++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/src/check_asm.cpp b/src/check_asm.cpp
index 89ce30a3c..124d65a3b 100644
--- a/src/check_asm.cpp
+++ b/src/check_asm.cpp
@@ -940,6 +940,8 @@ gb_internal void check_mnemonic(AsmCtx *asm_ctx, CheckerContext *ctx, Entity *tm
 		tmpl_entity->AsmTemplate.clobber_memory |= clobber.implies_clobber_memory();
 		tmpl_entity->AsmTemplate.is_volatile    |= clobber.implies_side_effects();
 
+		tmpl_entity->AsmTemplate.has_observable_side_effect |= clobber.implies_side_effects() != 0;
+		tmpl_entity->AsmTemplate.has_observable_side_effect |= clobber.writes_mem;
 
 		u16 pinned_mask = 0;
 		for (auto const &ed : tmpl_entity->AsmTemplate.decls) {
@@ -1612,7 +1614,6 @@ gb_internal void check_asm_template(AsmCtx *asm_ctx, CheckerContext *ctx, Entity
 		}
 	}
 
-
 	Array<Operand> operands = {};
 	operands.allocator = heap_allocator();
 	array_reserve(&operands, 16);
@@ -1766,6 +1767,28 @@ gb_internal void check_asm_template(AsmCtx *asm_ctx, CheckerContext *ctx, Entity
 	if (previous_prefix != 0) {
 		error(previous_prefix_instr, "A prefix must be immediately followed by an instruction, but the template ended");
 	}
+
+	for (auto const &ed : ate->decls) {
+		if (!(ed.param_group == AsmTemplateEntityDeclParamGroup_Output && ed.pin.len != 0)) {
+			continue;
+		}
+		u16 bit = asm_ctx->clobber_bit_for_reg_name(ed.pin);
+		if (bit && (defined_regs & bit) == 0 && straight_line) {
+			error(ed.entity->token,
+			      "Output '%.*s' is pinned to %%%.*s but nothing in this template writes it",
+			      LIT(ed.entity->token.string), LIT(ed.pin));
+		}
+	}
+
+	GB_ASSERT(entity->kind == Entity_AsmTemplate);
+	if (results->Tuple.variables.count == 0 && !entity->AsmTemplate.is_volatile &&
+	    !entity->AsmTemplate.clobber_memory &&
+	    entity->AsmTemplate.has_observable_side_effect) {
+		warning(entity->token,
+		        "This asm template has an observable effect but declares no outputs "
+		        "and does not #volatile in the specification block; it may be optimized away. "
+		        "Please add #volatile if the effect is intended.");
+	}
 }
 
 gb_internal void check_asm_template_from_entity(CheckerContext *c, Entity *e, DeclInfo *d) {
diff --git a/src/entity.cpp b/src/entity.cpp
index 8248515f4..e21ec4483 100644
--- a/src/entity.cpp
+++ b/src/entity.cpp
@@ -346,6 +346,9 @@ struct Entity {
 			bool is_volatile;
 			bool is_align_stack;
 
+			bool has_observable_side_effect;
+
+
 			bool      clobber_flags;
 			bool      clobber_memory;
 			StringSet clobber_registers_set;