Division by zero in bigNatRemWord

Summary

Out of luck I realized the following warning which I can only see if I run the command manually but not during a build done via hadrian. Thus I'm not sure whether this is a problem or not. Can someone confirm this?

ghc -Wall -fdiagnostics-color=never -hisuf hi -osuf o -hcsuf hc -static -hide-all-packages -no-user-package-db '-package-db _build/stage0/lib/package.conf.d' '-this-unit-id ghc-bignum-1.0' '-package-id ghc-prim-0.6.1' -i -i/ghc/_build/stage0/libraries/ghc-bignum/build -i/ghc/_build/stage0/libraries/ghc-bignum/build/autogen -i/ghc/libraries/ghc-bignum/src/ -I_build/stage0/lib -I_build/stage0/libraries/ghc-bignum/build -I_build/stage0/libraries/ghc-bignum/build/include/ -I_build/stage0/libraries/ghc-bignum/build/include -Ilibraries/ghc-bignum/include/ -Ilibraries/ghc-bignum/include -I/usr/lib64/ghc-8.10.1/include -I_build/stage0/lib -optc-I_build/stage0/lib -optP-include -optP_build/stage0/libraries/ghc-bignum/build/autogen/cabal_macros.h -optP-DBIGNUM_GMP -outputdir _build/stage0/libraries/ghc-bignum/build -optc-Wno-error=inline -optP-Wno-nonportable-include-path -c libraries/ghc-bignum/src/GHC/Num/BigNat.hs -o _build/stage0/libraries/ghc-bignum/build/GHC/Num/BigNat.o -O0 -H64m -Wall -this-unit-id ghc-bignum -XHaskell2010 -fno-warn-deprecated-flags -fforce-recomp 
/tmp/ghc1307255_0/ghc_3.hc: In function ‘ghczmbignum_GHCziNumziBigNat_bigNatRemWordzh_entry’:

/tmp/ghc1307255_0/ghc_3.hc:5818:15: error:
     warning: division by zero [-Wdiv-by-zero]
     5818 | _c4du = 0x1UL % 0x0UL;
          |               ^
     |
5818 | _c4du = 0x1UL % 0x0UL;
     |               ^
/tmp/ghc1307255_0/ghc_3.hc: In function ‘_s3CA’:

/tmp/ghc1307255_0/ghc_3.hc:14926:15: error:
     warning: division by zero [-Wdiv-by-zero]
      |
14926 | _c4Sa = 0x1UL % 0x0UL;
      |               ^
14926 | _c4Sa = 0x1UL % 0x0UL;
      |               ^

Environment

• GHC version used: 8.10.1

Optional:

• Operating System: Fedora 32
• System Architecture: s390x

added needs triage label

Another nice catch. I think that the following patch should fix it:

--- libraries/ghc-bignum/src/GHC/Num/BigNat.hs
+++ libraries/ghc-bignum/src/GHC/Num/BigNat.hs
@@ -531,7 +531,7 @@ bigNatQuotWord a (W# b) = bigNatQuotWord# a b
 --    b /= 0
 bigNatRemWord# :: BigNat -> Word# -> Word#
 bigNatRemWord# a b
-   | 0## <- b       = 1## `remWord#` 0##
+   | 0## <- b       = case divByZero of _ -> 0##
    | 1## <- b       = 0##
    | bigNatIsZero a = 0##
    | True           = inline bignat_rem_word a b

added ghc build failure integer-gmp labels

added ghc-bignum label and removed integer-gmp label

Can confirm, your patch fixes the warning for me. Thanks!

I'm just wondering why the warning didn't show up while using hadrian.

Can confirm, your patch fixes the warning for me. Thanks!

Great :)

I'm just wondering why the warning didn't show up while using hadrian.

Indeed, it didn't appear at all on CI either. Are you validating an unregisterised build?

Are you validating an unregisterised build?

I typically build an unregisterised and registerised version with --flavour=validate. I just saw the warning pop up on the console while having a look at #18358 (closed) but don't remember anymore if it was an unregisterised build or not.

mentioned in merge request !3554 (closed)

added Phigh Tbug labels and removed needs triage label

changed milestone to %9.0.1

mentioned in commit 9c68020c

mentioned in commit aed83188

mentioned in commit e5fb1b52

mentioned in commit bbc6c375

mentioned in commit a403eb91

mentioned in commit 1b3d13b6

Fixed by !3554 (closed)

closed

mentioned in commit 72d33e73

mentioned in merge request !4344 (closed)

mentioned in commit b9fd0f0f

mentioned in commit 8da38465

mentioned in commit d3e5a77c