Looks like my idea of what a "borrow" is was wrong. The PoP says:

 "If the resulting subtraction results in a carry out of bit zero, a value
 of one is placed in the corresponding element of the first operand;
 otherwise, a value of zero is placed in the corresponding element"

As clarified by Richard, all we have to do is invert the result.

Fixes: 1ee2d7ba72f6 ("s390x/tcg: Implement VECTOR SUBTRACT COMPUTE BORROW INDICATION")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 target/s390x/translate_vx.inc.c | 7 ++++---
 target/s390x/vec_int_helper.c   | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/target/s390x/translate_vx.inc.c b/target/s390x/translate_vx.inc.c
index 5ce7bfb0af..6032021d82 100644
--- a/target/s390x/translate_vx.inc.c
+++ b/target/s390x/translate_vx.inc.c
@@ -2132,12 +2132,12 @@ static DisasJumpType op_vs(DisasContext *s, DisasOps *o)
 
 static void gen_scbi_i32(TCGv_i32 d, TCGv_i32 a, TCGv_i32 b)
 {
-    tcg_gen_setcond_i32(TCG_COND_LTU, d, a, b);
+    tcg_gen_setcond_i32(TCG_COND_GEU, d, a, b);
 }
 
 static void gen_scbi_i64(TCGv_i64 d, TCGv_i64 a, TCGv_i64 b)
 {
-    tcg_gen_setcond_i64(TCG_COND_LTU, d, a, b);
+    tcg_gen_setcond_i64(TCG_COND_GEU, d, a, b);
 }
 
 static void gen_scbi2_i64(TCGv_i64 dl, TCGv_i64 dh, TCGv_i64 al,
@@ -2151,7 +2151,8 @@ static void gen_scbi2_i64(TCGv_i64 dl, TCGv_i64 dh, TCGv_i64 al,
     tcg_gen_andi_i64(th, th, 1);
     tcg_gen_sub2_i64(tl, th, ah, zero, th, zero);
     tcg_gen_sub2_i64(tl, th, tl, th, bh, zero);
-    tcg_gen_andi_i64(dl, th, 1);
+    /* "invert" the result: -1 -> 0; 0 -> 1 */
+    tcg_gen_addi_i64(dl, th, 1);
     tcg_gen_mov_i64(dh, zero);
 
     tcg_temp_free_i64(th);
diff --git a/target/s390x/vec_int_helper.c b/target/s390x/vec_int_helper.c
index d38405848f..0d6bc13dd6 100644
--- a/target/s390x/vec_int_helper.c
+++ b/target/s390x/vec_int_helper.c
@@ -593,7 +593,7 @@ void HELPER(gvec_vscbi##BITS)(void *v1, const void *v2, const void *v3,        \
         const uint##BITS##_t a = s390_vec_read_element##BITS(v2, i);           \
         const uint##BITS##_t b = s390_vec_read_element##BITS(v3, i);           \
                                                                                \
-        s390_vec_write_element##BITS(v1, i, a < b);                            \
+        s390_vec_write_element##BITS(v1, i, a >= b);                           \
     }                                                                          \
 }
 DEF_VSCBI(8)
-- 
2.21.0

On 10/21/19 1:57 AM, David Hildenbrand wrote:
> Looks like my idea of what a "borrow" is was wrong. The PoP says:
> 
>  "If the resulting subtraction results in a carry out of bit zero, a value
>  of one is placed in the corresponding element of the first operand;
>  otherwise, a value of zero is placed in the corresponding element"
> 
> As clarified by Richard, all we have to do is invert the result.
> 
> Fixes: 1ee2d7ba72f6 ("s390x/tcg: Implement VECTOR SUBTRACT COMPUTE BORROW INDICATION")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  target/s390x/translate_vx.inc.c | 7 ++++---
>  target/s390x/vec_int_helper.c   | 2 +-
>  2 files changed, 5 insertions(+), 4 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~