id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7574	Register allocator chokes on certain branches with literals	thoughtpolice	thoughtpolice	"
While running the test for #7571 (test is in #7573,) under '''WAY=normal''' instead of '''WAY=llvm''', I encountered this bug in the native backend:

{{{

=====> T7571(normal) 6 of 6 [0, 0, 0]
cd . && '/Users/a/code/haskell/ghc/inplace/bin/ghc-stage2' -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -fno-ghci-history -c T7571.cmm   -no-hs-main   >T7571.comp.stderr 2>&1
Compile failed (status 256) errors were:
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20130113 for x86_64-apple-darwin):
	allocateRegsAndSpill: Cannot read from uninitialized register
    %vI_c7

}}}

The test in question is:

{{{

#include ""Cmm.h""

testLiteralBranch (W_ dst, W_ src)
{
  if (1) {
    prim %memcpy(dst, src, 1024, 4);
  } else {
    prim %memcpy(dst, src, 512, 8);
  }
  return ();
}

}}}

If you comment out the branch conditionals, the test passes, so clearly something fishy is going on here. The test also fails if you change the condition to ```if (1 == 1)```

I have absolutely no idea how this did not trip the profiling-based build in StgStdThunks.cmm, like in the LLVM build c.f. #7571"	bug	new	high	7.8.1	Compiler (NCG)	7.7		ncg, codegen	pho@… mle+hs@… roma@…	Unknown/Multiple	Unknown/Multiple	Compile-time crash	Unknown		7573		#7571,#7534