Introduce architecture-specific IR lowering stage

This commit adds a clean separation between target-independent IR
optimization and architecture-specific code generation by introducing
an arch-lowering stage that runs after peephole optimization.

Author: jserv

src/arch-lower.c

@@ -0,0 +1,62 @@
+/*
+ * shecc - Architecture-specific IR lowering stage
+ *
+ * Introduces a minimal arch-lowering boundary that applies target-specific
+ * tweaks to phase-2 IR (ph2_ir) before final code generation. This keeps
+ * backends simpler by moving decisions that depend on CFG shape or target
+ * quirks out of emit-time where possible.
+ */
+
+#include "../config"
+#include "defs.h"
+
+/* ARM-specific lowering:
+ * - Mark detached conditional branches so codegen can decide between
+ *   short/long forms without re-deriving CFG shape.
+ */
+void arm_lower(void)
+{
+    for (func_t *func = FUNC_LIST.head; func; func = func->next) {
+        for (basic_block_t *bb = func->bbs; bb; bb = bb->rpo_next) {
+            for (ph2_ir_t *insn = bb->ph2_ir_list.head; insn;
+                 insn = insn->next) {
+                /* Mark branches that don't fall through to next block */
+                if (insn->op == OP_branch) {
+                    /* In SSA, we index 'else_bb' first, and then 'then_bb' */
+                    insn->is_branch_detached = (insn->else_bb != bb->rpo_next);
+                }
+            }
+        }
+    }
+}
+
+/* RISC-V-specific lowering:
+ * - Mark detached conditional branches
+ * - Future: prepare for RISC-V specific patterns
+ */
+void riscv_lower(void)
+{
+    for (func_t *func = FUNC_LIST.head; func; func = func->next) {
+        for (basic_block_t *bb = func->bbs; bb; bb = bb->rpo_next) {
+            for (ph2_ir_t *insn = bb->ph2_ir_list.head; insn;
+                 insn = insn->next) {
+                /* Mark branches that don't fall through to next block */
+                if (insn->op == OP_branch)
+                    insn->is_branch_detached = (insn->else_bb != bb->rpo_next);
+            }
+        }
+    }
+}
+
+/* Entry point: dispatch to the active architecture. */
+void arch_lower(void)
+{
+#if ELF_MACHINE == 0x28 /* ARM */
+    arm_lower();
+#elif ELF_MACHINE == 0xf3 /* RISC-V */
+    riscv_lower();
+#else
+    /* Unknown architecture: keep behavior as-is. */
+    (void) 0;
+#endif
+}

src/arm-codegen.c

@@ -172,11 +172,7 @@ void cfg_flatten(void)
                     flatten_ir->src1 = bb->belong_to->stack_size;
                 }
 
-                if (insn->op == OP_branch) {
-                    /* In SSA, we index 'else_bb' first, and then 'then_bb' */
-                    if (insn->else_bb != bb->rpo_next)
-                        flatten_ir->is_branch_detached = true;
-                }
+                /* Branch detachment is determined in the arch-lowering stage */
 
                 update_elf_offset(flatten_ir);
             }

src/globals.c

@@ -615,6 +615,9 @@ ph2_ir_t *add_ph2_ir(opcode_t op)
 {
     ph2_ir_t *ph2_ir = arena_alloc(BB_ARENA, sizeof(ph2_ir_t));
     ph2_ir->op = op;
+    /* Set safe defaults; arch-lowering may annotate later */
+    ph2_ir->next = NULL;
+    ph2_ir->is_branch_detached = 0;
     return add_existed_ph2_ir(ph2_ir);
 }
 

src/main.c

@@ -37,6 +37,9 @@
 /* Peephole optimization */
 #include "peephole.c"
 
+/* Arch-specific IR lowering boundary */
+#include "arch-lower.c"
+
 /* Machine code generation. support ARMv7-A and RV32I */
 #include "codegen.c"
 
@@ -103,6 +106,9 @@ int main(int argc, char *argv[])
 
     peephole();
 
+    /* Apply arch-specific IR tweaks before final codegen */
+    arch_lower();
+
     /* flatten CFG to linear instruction */
     cfg_flatten();
 

src/reg-alloc.c

@@ -55,6 +55,9 @@ ph2_ir_t *bb_add_ph2_ir(basic_block_t *bb, opcode_t op)
 {
     ph2_ir_t *n = arena_alloc(BB_ARENA, sizeof(ph2_ir_t));
     n->op = op;
+    /* Ensure deterministic defaults for newly created IR nodes */
+    n->next = NULL;            /* well-formed singly linked list */
+    n->is_branch_detached = 0; /* arch-lowering will set for branches */
 
     if (!bb->ph2_ir_list.head)
         bb->ph2_ir_list.head = n;