Skip s_mask computation for logical right shift.

Cc: qemu-stable@nongnu.org
Fixes: 93a967fbb57 ("tcg/optimize: Propagate sign info for shifting")
Reported-by: Jacob Young <jacobly@ziglang.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/optimize.c                     | 11 ++++++++++-
 tests/tcg/i386/test-i386-opt-shr.c | 18 ++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/tcg/i386/test-i386-opt-shr.c

diff --git a/tcg/optimize.c b/tcg/optimize.c
index 2960c0f3b7..8a394acf00 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -2666,8 +2666,17 @@ static bool fold_shift(OptContext *ctx, TCGOp *op)
 
         z_mask = do_constant_folding(op->opc, ctx->type, z_mask, sh);
         o_mask = do_constant_folding(op->opc, ctx->type, o_mask, sh);
-        s_mask = do_constant_folding(op->opc, ctx->type, s_mask, sh);
 
+        if (op->opc == INDEX_op_shr) {
+            /*
+             * Logical right shift will force the sign bit zero.
+             * Don't bother computing s_mask and let fold_masks
+             * recompute from z_mask.
+             */
+            return fold_masks_zo(ctx, op, z_mask, o_mask);
+        }
+
+        s_mask = do_constant_folding(op->opc, ctx->type, s_mask, sh);
         return fold_masks_zos(ctx, op, z_mask, o_mask, s_mask);
     }
 
diff --git a/tests/tcg/i386/test-i386-opt-shr.c b/tests/tcg/i386/test-i386-opt-shr.c
new file mode 100644
index 0000000000..b1ba64689e
--- /dev/null
+++ b/tests/tcg/i386/test-i386-opt-shr.c
@@ -0,0 +1,18 @@
+#include <assert.h>
+
+int main()
+{
+#ifndef __x86_64__
+    char test;
+
+    asm("movw $0x4000, %%ax\n\t"
+        "addw %%ax, %%ax\n\t"
+        "cwtl\n\t"
+        "shrl %%eax\n\t"
+        "cmpw $-0x3fff, %%ax\n\t"
+        "setnl %%al"
+        : "=a"(test));
+    assert(!test);
+#endif
+    return 0;
+}
-- 
2.43.0