[gcc r12-6967] [nvptx] Handle nop in prevent_branch_around_nothing

[gcc r12-6967] [nvptx] Handle nop in prevent_branch_around_nothing

[Tom de Vries]

https://gcc.gnu.org/g:456de10c549379b74d4858f00d4b8817035a73fc

commit r12-6967-g456de10c549379b74d4858f00d4b8817035a73fc
Author: Tom de Vries <tdevries@suse.de>
Date:   Sun Jan 23 06:42:24 2022 +0100

    [nvptx] Handle nop in prevent_branch_around_nothing
    
    When running libgomp test-case reduction-7.c on an nvptx accelerator
    (T400, driver version 470.86) and GOMP_NVPTX_JIT=-O0, I run into:
    ...
    reduction-7.exe:reduction-7.c:312: v_p_2: \
      Assertion `out[j * 32 + i] == (i + j) * 2' failed.
    FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/reduction-7.c \
      -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none \
      -O0  execution test
    ...
    
    During investigation I found ptx code like this:
    ...
    @ %r163 bra $L262;
    $L262:
    ...
    
    There's a known problem with executing this type of code, and a workaround is
    in place to address this: prevent_branch_around_nothing.  The workaround does
    not trigger though because it doesn't handle the nop insn.
    
    Fix this by handling the nop insn in prevent_branch_around_nothing.
    
    Tested libgomp on x86_64 with nvptx accelerator.
    
    gcc/ChangeLog:
    
    2022-01-27  Tom de Vries  <tdevries@suse.de>
    
            PR target/100428
            * config/nvptx/nvptx.cc (prevent_branch_around_nothing): Handle nop
            insn.

Diff:
---
 gcc/config/nvptx/nvptx.cc | 1 +
 1 file changed, 1 insertion(+)

diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index ceea4d3a093..262e8f9cc1b 100644
--- a/gcc/config/nvptx/nvptx.cc
+++ b/gcc/config/nvptx/nvptx.cc
@@ -5103,6 +5103,7 @@ prevent_branch_around_nothing (void)
 	    case CODE_FOR_nvptx_forked:
 	    case CODE_FOR_nvptx_joining:
 	    case CODE_FOR_nvptx_join:
+	    case CODE_FOR_nop:
 	      continue;
 	    default:
 	      seen_label = NULL;