From e6f1bfb21121b5aaf6f5ff339157c0d55e8b9fe2 Mon Sep 17 00:00:00 2001 From: "Paul A. Clarke" Date: Mon, 19 Aug 2019 12:43:21 -0500 Subject: [PATCH] ppc: conform to processor User's Manual for xscvdpspn The POWER8 and POWER9 User's Manuals specify the implementation behavior for what the ISA leaves "undefined" behavior for the xscvdpspn and xscvdpsp instructions. This patch corrects the QEMU implementation to match the hardware implementation for that case. ISA 3.0B has xscvdpspn leaving its result in word 0 of the target register, with the other words of the target register left "undefined". The User's Manuals specify: VSX scalar convert from double-precision to single-precision (xscvdpsp, xscvdpspn). VSR[32:63] is set to VSR[0:31]. So, words 0 and 1 both contain the result. Note: this is important because GCC as of version 8 or so, assumes and takes advantage of this behavior to optimize the following sequence: xscvdpspn vs0,vs1 mffprwz r8,f0 ISA 3.0B has xscvdpspn leaving its result in word 0 of the target register, and mffprwz expecting its input to come from word 1 of the source register. This sequence fails with QEMU, as a shift is required between those two instructions. However, since the hardware splats the result to both words 0 and 1 of its output register, the shift is not necessary. Expect a future revision of the ISA to specify this behavior. Signed-off-by: Paul A. Clarke v2 - Splitting patch "ppc: Three floating point fixes"; this is just one part. - Updated commit message to clarify behavior is documented in User's Manuals. - Updated commit message to correct which words are in output and source of xscvdpspn and mffprz. - No source changes to this part of the original patch. Message-Id: <1566236601-22954-1-git-send-email-pc@us.ibm.com> Signed-off-by: David Gibson --- target/ppc/fpu_helper.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c index 5611cf0156..23b9c97439 100644 --- a/target/ppc/fpu_helper.c +++ b/target/ppc/fpu_helper.c @@ -2871,10 +2871,14 @@ void helper_xscvqpdp(CPUPPCState *env, uint32_t opcode, uint64_t helper_xscvdpspn(CPUPPCState *env, uint64_t xb) { + uint64_t result; + float_status tstat = env->fp_status; set_float_exception_flags(0, &tstat); - return (uint64_t)float64_to_float32(xb, &tstat) << 32; + result = (uint64_t)float64_to_float32(xb, &tstat); + /* hardware replicates result to both words of the doubleword result. */ + return (result << 32) | result; } uint64_t helper_xscvspdpn(CPUPPCState *env, uint64_t xb) -- 2.11.0