Mesa (master): gallivm: Workaround http://llvm.org/PR18600

[Jose Fonseca]

Module: Mesa
Branch: master
Commit: 2eddf91faf4abf0b3d7b81a0a486fd46d5acd1ea
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=2eddf91faf4abf0b3d7b81a0a486fd46d5acd1ea

Author: José Fonseca <jfonseca at vmware.com>
Date:   Fri Jan 24 14:25:46 2014 +0000

gallivm: Workaround http://llvm.org/PR18600

We have code generation paths that carry out swizzles of AoS vectors via
bitwise shifts, as these tend to generate more efficient code than
straightforward byte shuffles.  But when the input is a constant the
additional bitwise arithmetic operations somehow don't really get
constant propagated properly, evenutally causing assertion failure in
InstCombine pass.

Therefore avoid the bug by using the trivial shuffles for constant
inputs.

Although the sample LLVM IR can cause a crash with any LLVM version,
this was only seen in practice with LLVM 3.2.

Reviewed-by: Matthew McClure <mcclurem at vmware.com>
Reviewed-by: Roland Scheidegger <sroland at vmware.com>

---

 src/gallium/auxiliary/gallivm/lp_bld_swizzle.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/gallium/auxiliary/gallivm/lp_bld_swizzle.c b/src/gallium/auxiliary/gallivm/lp_bld_swizzle.c
index 79116bc..9557e1c 100644
--- a/src/gallium/auxiliary/gallivm/lp_bld_swizzle.c
+++ b/src/gallium/auxiliary/gallivm/lp_bld_swizzle.c
@@ -180,7 +180,8 @@ lp_build_swizzle_scalar_aos(struct lp_build_context *bld,
    /* XXX: SSE3 has PSHUFB which should be better than bitmasks, but forcing
     * using shuffles here actually causes worst results. More investigation is
     * needed. */
-   if (type.width >= 16) {
+   if (LLVMIsConstant(a) ||
+       type.width >= 16) {
       /*
        * Shuffle.
        */
@@ -398,7 +399,8 @@ lp_build_swizzle_aos(struct lp_build_context *bld,
       }
    }
 
-   if (type.width >= 16) {
+   if (LLVMIsConstant(a) ||
+       type.width >= 16) {
       /*
        * Shuffle.
        */