Re: [PATCH v2] cputlb: Make store_helper less fragile to compiler optimizations

[Richard Henderson <richard.henderson@linaro.org>]

On 7/24/20 12:51 PM, Shu-Chun Weng wrote:
> This change has no functional change.
> 
> There is a potential link error with "undefined symbol:
> qemu_build_not_reached" due to how `store_helper` is structured.
> This does not produce at current QEMU head, but was reproducible at
> v4.2.0 with `clang-10 -O2 -fexperimental-new-pass-manager`.

Thanks for the hint -- so far I had not been able to reproduce the
problem with any of clang 10, 11, or head (12), with default options.

> The current function structure is:
> 
>     inline QEMU_ALWAYSINLINE
>     store_memop() {
>         switch () {
>             ...
>         default:
>             qemu_build_not_reached();
>         }
>     }
>     inline QEMU_ALWAYSINLINE
>     store_helper() {
>         ...
>         if (span_two_pages_or_io) {
>             ...
>             helper_rst_stb_mmu();
>         }
>         store_memop();
>     }
>     helper_rst_stb_mmu() {
>         store_helper();
>     }
...
> The new structure is:
> 
>     inline QEMU_ALWAYSINLINE
>     store_memop() {
>         switch () {
>             ...
>         default:
>             qemu_build_not_reached();
>         }
>     }
>     inline QEMU_ALWAYSINLINE
>     store_helper_size_aligned()() {
>         ...
>         if (span_two_pages_or_io) {
>             return false;
>         }
>         store_memoop();
>         return true;
>     }
>     inline QEMU_ALWAYSINLINE
>     store_helper() {
>         if (store_helper_size_aligned() {
>             return;
>         }
>         helper_rst_stb_mmu();
>     }
>     helper_rst_stb_mmu() {
>         store_helper_size_aligned()();
>     }

Reasonable, I guess.

I did some experimenting though, and if I pull out the unaligned
portion into a noinline function, I can save about 6k code size.

Thoughts?


r~


diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
index 5698292749..7e603d6666 100644
--- a/accel/tcg/cputlb.c
+++ b/accel/tcg/cputlb.c
@@ -2009,6 +2009,80 @@ store_memop(void *haddr, uint64_t val, MemOp op)
     }
 }
 
+static void __attribute__((noinline))
+store_helper_unaligned(CPUArchState *env, target_ulong addr, uint64_t val,
+                       uintptr_t retaddr, size_t size, uintptr_t mmu_idx,
+                       bool big_endian)
+{
+    const size_t tlb_off = offsetof(CPUTLBEntry, addr_write);
+    uintptr_t index, index2;
+    CPUTLBEntry *entry, *entry2;
+    target_ulong page2, tlb_addr, tlb_addr2;
+    TCGMemOpIdx oi;
+    size_t size2;
+    int i;
+
+    /*
+     * Ensure the second page is in the TLB.  Note that the first page
+     * is already guaranteed to be filled, and that the second page
+     * cannot evict the first.
+     */
+    page2 = (addr + size) & TARGET_PAGE_MASK;
+    size2 = (addr + size) & ~TARGET_PAGE_MASK;
+    index2 = tlb_index(env, mmu_idx, page2);
+    entry2 = tlb_entry(env, mmu_idx, page2);
+
+    tlb_addr2 = tlb_addr_write(entry2);
+    if (!tlb_hit_page(tlb_addr2, page2)) {
+        if (!victim_tlb_hit(env, mmu_idx, index2, tlb_off, page2)) {
+            tlb_fill(env_cpu(env), page2, size2, MMU_DATA_STORE,
+                     mmu_idx, retaddr);
+            index2 = tlb_index(env, mmu_idx, page2);
+            entry2 = tlb_entry(env, mmu_idx, page2);
+        }
+        tlb_addr2 = tlb_addr_write(entry2);
+    }
+
+    index = tlb_index(env, mmu_idx, addr);
+    entry = tlb_entry(env, mmu_idx, addr);
+    tlb_addr = tlb_addr_write(entry);
+
+    /*
+     * Handle watchpoints.  Since this may trap, all checks
+     * must happen before any store.
+     */
+    if (unlikely(tlb_addr & TLB_WATCHPOINT)) {
+        cpu_check_watchpoint(env_cpu(env), addr, size - size2,
+                             env_tlb(env)->d[mmu_idx].iotlb[index].attrs,
+                             BP_MEM_WRITE, retaddr);
+    }
+    if (unlikely(tlb_addr2 & TLB_WATCHPOINT)) {
+        cpu_check_watchpoint(env_cpu(env), page2, size2,
+                             env_tlb(env)->d[mmu_idx].iotlb[index2].attrs,
+                             BP_MEM_WRITE, retaddr);
+    }
+
+    /*
+     * XXX: not efficient, but simple.
+     * This loop must go in the forward direction to avoid issues
+     * with self-modifying code in Windows 64-bit.
+     */
+    oi = make_memop_idx(MO_UB, mmu_idx);
+    if (big_endian) {
+        for (i = 0; i < size; ++i) {
+            /* Big-endian extract.  */
+            uint8_t val8 = val >> (((size - 1) * 8) - (i * 8));
+            helper_ret_stb_mmu(env, addr + i, val8, oi, retaddr);
+        }
+    } else {
+        for (i = 0; i < size; ++i) {
+            /* Little-endian extract.  */
+            uint8_t val8 = val >> (i * 8);
+            helper_ret_stb_mmu(env, addr + i, val8, oi, retaddr);
+        }
+    }
+}
+
 static inline void QEMU_ALWAYS_INLINE
 store_helper(CPUArchState *env, target_ulong addr, uint64_t val,
              TCGMemOpIdx oi, uintptr_t retaddr, MemOp op)
@@ -2097,64 +2171,9 @@ store_helper(CPUArchState *env, target_ulong addr, uint64_t val,
     if (size > 1
         && unlikely((addr & ~TARGET_PAGE_MASK) + size - 1
                      >= TARGET_PAGE_SIZE)) {
-        int i;
-        uintptr_t index2;
-        CPUTLBEntry *entry2;
-        target_ulong page2, tlb_addr2;
-        size_t size2;
-
     do_unaligned_access:
-        /*
-         * Ensure the second page is in the TLB.  Note that the first page
-         * is already guaranteed to be filled, and that the second page
-         * cannot evict the first.
-         */
-        page2 = (addr + size) & TARGET_PAGE_MASK;
-        size2 = (addr + size) & ~TARGET_PAGE_MASK;
-        index2 = tlb_index(env, mmu_idx, page2);
-        entry2 = tlb_entry(env, mmu_idx, page2);
-        tlb_addr2 = tlb_addr_write(entry2);
-        if (!tlb_hit_page(tlb_addr2, page2)) {
-            if (!victim_tlb_hit(env, mmu_idx, index2, tlb_off, page2)) {
-                tlb_fill(env_cpu(env), page2, size2, MMU_DATA_STORE,
-                         mmu_idx, retaddr);
-                index2 = tlb_index(env, mmu_idx, page2);
-                entry2 = tlb_entry(env, mmu_idx, page2);
-            }
-            tlb_addr2 = tlb_addr_write(entry2);
-        }
-
-        /*
-         * Handle watchpoints.  Since this may trap, all checks
-         * must happen before any store.
-         */
-        if (unlikely(tlb_addr & TLB_WATCHPOINT)) {
-            cpu_check_watchpoint(env_cpu(env), addr, size - size2,
-                                 env_tlb(env)->d[mmu_idx].iotlb[index].attrs,
-                                 BP_MEM_WRITE, retaddr);
-        }
-        if (unlikely(tlb_addr2 & TLB_WATCHPOINT)) {
-            cpu_check_watchpoint(env_cpu(env), page2, size2,
-                                 env_tlb(env)->d[mmu_idx].iotlb[index2].attrs,
-                                 BP_MEM_WRITE, retaddr);
-        }
-
-        /*
-         * XXX: not efficient, but simple.
-         * This loop must go in the forward direction to avoid issues
-         * with self-modifying code in Windows 64-bit.
-         */
-        for (i = 0; i < size; ++i) {
-            uint8_t val8;
-            if (memop_big_endian(op)) {
-                /* Big-endian extract.  */
-                val8 = val >> (((size - 1) * 8) - (i * 8));
-            } else {
-                /* Little-endian extract.  */
-                val8 = val >> (i * 8);
-            }
-            helper_ret_stb_mmu(env, addr + i, val8, oi, retaddr);
-        }
+        store_helper_unaligned(env, addr, val, retaddr, size,
+                               mmu_idx, memop_big_endian(op));
         return;
     }
 
@@ -2162,8 +2181,9 @@ store_helper(CPUArchState *env, target_ulong addr, uint64_t val,
     store_memop(haddr, val, op);
 }
 
-void helper_ret_stb_mmu(CPUArchState *env, target_ulong addr, uint8_t val,
-                        TCGMemOpIdx oi, uintptr_t retaddr)
+void __attribute__((noinline))
+helper_ret_stb_mmu(CPUArchState *env, target_ulong addr, uint8_t val,
+                   TCGMemOpIdx oi, uintptr_t retaddr)
 {
     store_helper(env, addr, val, oi, retaddr, MO_UB);
 }
-- 
2.25.1