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[Committed] Bug fix to new wi::bitreverse_large function.

Message ID 009d01d99993$0a433840$1ec9a8c0$@nextmovesoftware.com
State Committed
Headers
Series [Committed] Bug fix to new wi::bitreverse_large function. |

Checks

Context Check Description
linaro-tcwg-bot/tcwg_gcc_build--master-arm fail Patch failed to apply
linaro-tcwg-bot/tcwg_gcc_build--master-aarch64 fail Patch failed to apply
linaro-tcwg-bot/tcwg_gcc_check--master-arm success Testing passed

Commit Message

Roger Sayle June 7, 2023, 10:54 p.m. UTC 
  Richard Sandiford was, of course, right to be warry of new code without
much test coverage.  Converting the nvptx backend to use the BITREVERSE
rtx infrastructure, has resulted in far more exhaustive testing and
revealed a subtle bug in the new wi::bitreverse implementation.  The
code needs to use HOST_WIDE_INT_1U (instead of 1) to avoid unintended
sign extension.

This patch has been tested on nvptx-none hosted on x86_64-pc-linux-gnu
(with a minor tweak to use BITREVERSE), where it fixes regressions of
the 32-bit test vectors in gcc.target/nvptx/brev-2.c and the 64-bit
test vectors in gcc.target/nvptx/brevll-2.c.  Committed as obvious.


2023-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * wide-int.cc (wi::bitreverse_large): Use HOST_WIDE_INT_1U to
        avoid sign extension/undefined behaviour when setting each bit.


Thanks,
Roger
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Patch

diff --git a/gcc/wide-int.cc b/gcc/wide-int.cc
index 24bdce2..ab92ee6 100644
--- a/gcc/wide-int.cc
+++ b/gcc/wide-int.cc
@@ -786,7 +786,7 @@  wi::bitreverse_large (HOST_WIDE_INT *val, const HOST_WIDE_INT *xval,
 	  unsigned int d = (precision - 1) - s;
 	  block = d / HOST_BITS_PER_WIDE_INT;
 	  offset = d & (HOST_BITS_PER_WIDE_INT - 1);
-          val[block] |= 1 << offset;
+          val[block] |= HOST_WIDE_INT_1U << offset;
 	}
     }