[Mesa-dev] [PATCH] gallivm: fix return opcode handling in main function of a shader

[sroland at vmware.com]

From: Roland Scheidegger <sroland at vmware.com>

If we're in some conditional or loop we must not return, or the code
after the condition is never executed.
(v2): And, we also can't just continue as nothing happened, since the
mask update code would later check if we actually have a mask, so we
need to remember that there was a return in main where we didn't exit
(to illustrate this, a ret in a if clause would cause a mask update
which is still ok as we're in a conditional, but after the endif the
mask update code would drop the mask hence bringing execution back to
pixels which should have their execution mask set to zero by the ret).
Thanks to Christoph Bumiller for figuring this out.

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=62357.

Note: This is a candidate for the stable branches.
---
 src/gallium/auxiliary/gallivm/lp_bld_tgsi.h     |    1 +
 src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c |   20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi.h b/src/gallium/auxiliary/gallivm/lp_bld_tgsi.h
index dac97c3..6e65e12 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi.h
+++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi.h
@@ -243,6 +243,7 @@ struct lp_exec_mask {
    struct lp_build_context *bld;
 
    boolean has_mask;
+   boolean ret_in_main;
 
    LLVMTypeRef int_vec_type;
 
diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
index 0dc26b5..965255a 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
@@ -73,6 +73,7 @@ static void lp_exec_mask_init(struct lp_exec_mask *mask, struct lp_build_context
 
    mask->bld = bld;
    mask->has_mask = FALSE;
+   mask->ret_in_main = FALSE;
    mask->cond_stack_size = 0;
    mask->loop_stack_size = 0;
    mask->call_stack_size = 0;
@@ -108,7 +109,7 @@ static void lp_exec_mask_update(struct lp_exec_mask *mask)
    } else
       mask->exec_mask = mask->cond_mask;
 
-   if (mask->call_stack_size) {
+   if (mask->call_stack_size || mask->ret_in_main) {
       mask->exec_mask = LLVMBuildAnd(builder,
                                      mask->exec_mask,
                                      mask->ret_mask,
@@ -117,7 +118,8 @@ static void lp_exec_mask_update(struct lp_exec_mask *mask)
 
    mask->has_mask = (mask->cond_stack_size > 0 ||
                      mask->loop_stack_size > 0 ||
-                     mask->call_stack_size > 0);
+                     mask->call_stack_size > 0 ||
+                     mask->ret_in_main);
 }
 
 static void lp_exec_mask_cond_push(struct lp_exec_mask *mask,
@@ -348,11 +350,23 @@ static void lp_exec_mask_ret(struct lp_exec_mask *mask, int *pc)
    LLVMBuilderRef builder = mask->bld->gallivm->builder;
    LLVMValueRef exec_mask;
 
-   if (mask->call_stack_size == 0) {
+   if (mask->cond_stack_size == 0 &&
+       mask->loop_stack_size == 0 &&
+       mask->call_stack_size == 0) {
       /* returning from main() */
       *pc = -1;
       return;
    }
+
+   if (mask->call_stack_size == 0) {
+      /*
+       * This requires special handling since we need to ensure
+       * we don't drop the mask even if we have no call stack
+       * (e.g. after a ret in a if clause after the endif)
+       */
+      mask->ret_in_main = TRUE;
+   }
+
    exec_mask = LLVMBuildNot(builder,
                             mask->exec_mask,
                             "ret");
-- 
1.7.9.5