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bitcast between long and double on x86-64 #1405

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sunfishcode opened this issue Dec 4, 2006 · 6 comments
Closed

bitcast between long and double on x86-64 #1405

sunfishcode opened this issue Dec 4, 2006 · 6 comments
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@lattner
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backend:X86 bugzilla Issues migrated from bugzilla compile-fail Use [accepts-invalid] and [rejects-valid] instead

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@sunfishcode
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Bugzilla Link 1033
Resolution FIXED
Resolved on Feb 22, 2010 12:48
Version trunk
OS Linux
CC @lattner

Extended Description

On x86-64 a bitcast from long to double or double to long results triggers an
llc abort. Among other things, this comes up in the expansion of copysign.

long %p(double %t) {
%u = bitcast double %t to long
ret long %u
}

double %q(long %t) {
%u = bitcast long %t to double
ret double %u
}

With llc -march=x86-64, this gets

Cannot yet select: 0x8813318: f64 = bit_convert 0x88135c8
@sunfishcode
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assigned to @lattner

@llvmbot
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llvmbot commented Dec 4, 2006

Mine.

@lattner
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lattner commented Dec 4, 2006

I think this is a codegen bug Reid,

-Chris

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llvmbot commented Dec 4, 2006

This one looks like yours Evan. It seems the TD for x86-64 isn't handling bit
casts of 64-bit things.

The test case for it has already been committed to Regression/CodeGen/X86.

Reid.

@lattner
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lattner commented Dec 5, 2006

The preferred fix for this is to expand it into movd/movq. However, Evan is busy with other things, so
this won't happen in the near term. In the short-term, this patch gets us correct, but suboptimal (goes
through memory) code:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20061204/040803.html

Testcase here: CodeGen/X86/bitcast.ll

-Chris

_test1:
subq $8, %rsp
movsd %xmm0, (%rsp)
movq (%rsp), %rax
addq $8, %rsp
ret
_test2:
subq $8, %rsp
movq %rdi, (%rsp)
movsd (%rsp), %xmm0
addq $8, %rsp
ret

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lattner commented Dec 5, 2006

Actually, MOVQ apparently doesn't do 64-bit GPR to SSE reg.

I implemented the movd case, compiling bitcast.ll::test3/test4 to:

_test3:
movd %xmm0, %eax
ret
_test4:
movd %edi, %xmm0
ret

Patch here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20061204/040806.html

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021
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