https://llvm.org/bugs/show_bug.cgi?id=26871
Bug ID: 26871
Summary: sitofp conversion to half-precision float generates
single-precision float without truncating
Product: libraries
Version: trunk
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: andres.noet...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
The following program converts -4095 (not precisely representable as
half-precision float) to a half-precision float, once at runtime and once at
compile time, and adds 1.0:
$ cat ../example.ll
define half @foo(i32 %x) {
%r = sitofp i32 %x to half
%rr = fadd half %r, 1.0
ret half %rr
}
define i1 @main() {
%x = sitofp i32 -4095 to half
%xx = fadd half %x, 1.0
%y = call half @foo(i32 -4095)
%rr = fcmp oeq half %xx, %y
ret i1 %rr
}
One would expect the result to be true (1) but the program returns false (0):
$ bin/llc -filetype=obj ../example.ll && clang ../example.o -o a.out &&
./a.out; echo $?
0
The reason seems to be that cvtsi2ssl is used to convert the integer to a
single-precision float but then the addition is done without truncating the
value to a half-precision float before extending it to a single-precision float
for the addition (Note: the problem seems to appear on multiple architectures
(x86, ARM, ...)):
$ bin/llc -o - ../example.ll
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 11
.globl _foo
.p2align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0:
pushq %rax
Ltmp0:
.cfi_def_cfa_offset 16
cvtsi2ssl %edi, %xmm0
movss %xmm0, 4(%rsp) ## 4-byte Spill
movl $15360, %edi ## imm = 0x3C00
callq ___extendhfsf2
addss 4(%rsp), %xmm0 ## 4-byte Folded Reload
popq %rax
retq
.cfi_endproc
.globl _main
.p2align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0:
pushq %rax
Ltmp1:
.cfi_def_cfa_offset 16
movl $-4095, %edi ## imm = 0xFFFFFFFFFFFFF001
callq _foo
callq ___truncsfhf2
movzwl %ax, %edi
callq ___extendhfsf2
movss %xmm0, 4(%rsp) ## 4-byte Spill
movl $60416, %edi ## imm = 0xEC00
callq ___extendhfsf2
movss 4(%rsp), %xmm1 ## 4-byte Reload
## xmm1 = mem[0],zero,zero,zero
cmpeqss %xmm0, %xmm1
movd %xmm1, %eax
andl $1, %eax
popq %rcx
retq
.cfi_endproc
.subsections_via_symbols
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