A bit more investigation: It looks like the bug happens in float->double conversion direction.
$ cat a.c #include <stdio.h> #include <stdint.h> int main() { volatile uint32_t i = 1; volatile float f; volatile double d; *(volatile uint32_t*)&f = i; d = f; printf("d = %#llx (expect 0x36a0000000000000)\n", *(volatile uint64_t*)&d); } $ powerpc-unknown-linux-gnu-gcc -O2 a.c -Wall -o a -fno-strict-aliasing -fno-stack-protector -static && qemu-ppc ./a d = 0x36b0000000080000 (expect 0x36a0000000000000) 10000400 <main>: 10000404: 39 20 00 01 li r9,1 ... 10000434: 91 21 00 10 stw r9,16(r1) ... 1000043c: c0 01 00 10 lfs f0,16(r1) 10000440: d8 01 00 08 stfd f0,8(r1) ... 10000448: 80 a1 00 08 lwz r5,8(r1) 1000044c: 80 c1 00 0c lwz r6,12(r1) ... 10000454: 48 02 01 ad bl 10020600 <___printf_chk> This is just lfs/stfd conversion. qemu does translates that pair if instructions into: $ ppc-linux-user/qemu-ppc -d in_asm,out_asm,op,op_opt /tmp/b/a ... IN: main ... 0x1000043c: c0010010 lfs f0, 0x10(r1) 0x10000440: d8010008 stfd f0, 8(r1) ... OP: ---- 1000043c movi_i32 tmp1,$0x10 add_i32 tmp0,r1,tmp1 qemu_ld_i32 tmp1,tmp0,beul,2 call todouble,$0x5,$1,tmp2,tmp1 st_i64 tmp2,env,$0x9198 'todouble' must be a 'uint64_t helper_todouble(uint32_t arg=0x1)' from: https://github.com/qemu/qemu/blob/master/target/ppc/fpu_helper.c#L55 -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1821444 Title: qemu-ppc (user) incorrectly translates float32 arithmetics Status in QEMU: New Bug description: I'm using qemu-3.1.0 (Gentoo). When I was running regression test suite via qemu-ppc for GHC I noticed a few uint32_t<->float32 failures I did not expect to encounter. Here is an example $ cat a.c #include <stdio.h> #include <stdint.h> int main() { volatile uint32_t i = 1; printf("0x1 = %e\n", *(volatile float*)&i); } $ powerpc-unknown-linux-gnu-gcc -O2 a.c -Wall -o a -fno-strict-aliasing -fno-stack-protector -static && ./a 0x1 = 2.802597e-45 $ scp a timberdoodle.ppc64.dev.gentoo.org:~/ a 100% 826KB 102.0KB/s 00:08 $ ssh timberdoodle.ppc64.dev.gentoo.org ./a 0x1 = 1.401298e-45 $ qemu-ppc ./a 0x1 = 2.802597e-45 Looks like off-by-one bit somewhere. I'm not sure if it's FPU instruction or some internals of printf() that are emulated incorrectly. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1821444/+subscriptions