Author: Sanjay Patel Date: 2020-09-30T13:28:43+02:00 New Revision: 60a25202a7dd1e00067fcfce512086ebf3788537
URL: https://github.com/llvm/llvm-project/commit/60a25202a7dd1e00067fcfce512086ebf3788537 DIFF: https://github.com/llvm/llvm-project/commit/60a25202a7dd1e00067fcfce512086ebf3788537.diff LOG: [APFloat] prevent NaN morphing into Inf on conversion (PR43907) We shift the significand right on a truncation, but that needs to be made NaN-safe: always set at least 1 bit in the significand. https://llvm.org/PR43907 See D88238 for the likely follow-up (but needs some plumbing fixes before it can proceed). Differential Revision: https://reviews.llvm.org/D87835 (cherry picked from commit e34bd1e0b03d20a506ada156d87e1b3a96d82fa2) Added: Modified: llvm/lib/Support/APFloat.cpp llvm/test/Transforms/ConstProp/cast.ll llvm/unittests/ADT/APFloatTest.cpp Removed: ################################################################################ diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp index 569cac790af9..362595d8f8b1 100644 --- a/llvm/lib/Support/APFloat.cpp +++ b/llvm/lib/Support/APFloat.cpp @@ -2242,6 +2242,21 @@ IEEEFloat::opStatus IEEEFloat::convert(const fltSemantics &toSemantics, if (!X86SpecialNan && semantics == &semX87DoubleExtended) APInt::tcSetBit(significandParts(), semantics->precision - 1); + // If we are truncating NaN, it is possible that we shifted out all of the + // set bits in a signalling NaN payload. But NaN must remain NaN, so some + // bit in the significand must be set (otherwise it is Inf). + // This can only happen with sNaN. Set the 1st bit after the quiet bit, + // so that we still have an sNaN. + // FIXME: Set quiet and return opInvalidOp (on convert of any sNaN). + // But this requires fixing LLVM to parse 32-bit hex FP or ignoring + // conversions while parsing IR. + if (APInt::tcIsZero(significandParts(), newPartCount)) { + assert(shift < 0 && "Should not lose NaN payload on extend"); + assert(semantics->precision >= 3 && "Unexpectedly narrow significand"); + assert(*losesInfo && "Missing payload should have set lost info"); + APInt::tcSetBit(significandParts(), semantics->precision - 3); + } + // gcc forces the Quiet bit on, which means (float)(double)(float_sNan) // does not give you back the same bits. This is dubious, and we // don't currently do it. You're really supposed to get diff --git a/llvm/test/Transforms/ConstProp/cast.ll b/llvm/test/Transforms/ConstProp/cast.ll index 8377df17b3a8..c07fa1295b42 100644 --- a/llvm/test/Transforms/ConstProp/cast.ll +++ b/llvm/test/Transforms/ConstProp/cast.ll @@ -38,3 +38,26 @@ define float @overflow_sitofp() { ret float %i } +; https://llvm.org/PR43907 - make sure that NaN doesn't morph into Inf. +; SNaN remains SNaN. + +define float @nan_f64_trunc() { +; CHECK-LABEL: @nan_f64_trunc( +; CHECK-NEXT: ret float 0x7FF4000000000000 +; + %f = fptrunc double 0x7FF0000000000001 to float + ret float %f +} + +; Verify again with a vector and diff erent destination type. +; SNaN remains SNaN (first two elements). +; QNaN remains QNaN (third element). +; Lower 42 bits of NaN source payload are lost. + +define <3 x half> @nan_v3f64_trunc() { +; CHECK-LABEL: @nan_v3f64_trunc( +; CHECK-NEXT: ret <3 x half> <half 0xH7D00, half 0xH7D00, half 0xH7E00> +; + %f = fptrunc <3 x double> <double 0x7FF0020000000000, double 0x7FF003FFFFFFFFFF, double 0x7FF8000000000001> to <3 x half> + ret <3 x half> %f +} diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp index b24b43d09a40..c798c95e05f6 100644 --- a/llvm/unittests/ADT/APFloatTest.cpp +++ b/llvm/unittests/ADT/APFloatTest.cpp @@ -1840,6 +1840,21 @@ TEST(APFloatTest, convert) { &losesInfo); EXPECT_TRUE(test.bitwiseIsEqual(X87QNaN)); EXPECT_FALSE(losesInfo); + + // The payload is lost in truncation, but we must retain NaN, so we set the bit after the quiet bit. + APInt payload(52, 1); + test = APFloat::getSNaN(APFloat::IEEEdouble(), false, &payload); + APFloat::opStatus status = test.convert(APFloat::IEEEsingle(), APFloat::rmNearestTiesToEven, &losesInfo); + EXPECT_EQ(0x7fa00000, test.bitcastToAPInt()); + EXPECT_TRUE(losesInfo); + EXPECT_EQ(status, APFloat::opOK); + + // The payload is lost in truncation. QNaN remains QNaN. + test = APFloat::getQNaN(APFloat::IEEEdouble(), false, &payload); + status = test.convert(APFloat::IEEEsingle(), APFloat::rmNearestTiesToEven, &losesInfo); + EXPECT_EQ(0x7fc00000, test.bitcastToAPInt()); + EXPECT_TRUE(losesInfo); + EXPECT_EQ(status, APFloat::opOK); } TEST(APFloatTest, PPCDoubleDouble) { _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
