[DAGCombine] (float)((int) f) --> ftrunc (PR36617)

This was originally committed at rL328921 and reverted at rL329920 to
investigate failures in Chrome. This time I've added to the ReleaseNotes
to warn users of the potential of exposing UB and let me repeat that
here for more exposure:

  Optimization of floating-point casts is improved. This may cause surprising
  results for code that is relying on undefined behavior. Code sanitizers can
  be used to detect affected patterns such as this:

    int main() {
      float x = 4294967296.0f;
      x = (float)((int)x);
      printf("junk in the ftrunc: %f\n", x);
      return 0;
    }

    $ clang -O1 ftrunc.c -fsanitize=undefined ; ./a.out
    ftrunc.c:5:15: runtime error: 4.29497e+09 is outside the range of 
                   representable values of type 'int'
    junk in the ftrunc: 0.000000


Original commit message:

fptosi / fptoui round towards zero, and that's the same behavior as ISD::FTRUNC,
so replace a pair of casts with the equivalent node. We don't have to account for
special cases (NaN, INF) because out-of-range casts are undefined.

Differential Revision: https://reviews.llvm.org/D44909

llvm-svn: 330437

llvm/docs/ReleaseNotes.rst

@@ -61,6 +61,26 @@ Non-comprehensive list of changes in this release
 * The optimization flag to merge constants (-fmerge-all-constants) is no longer
   applied by default.
 
+* Optimization of floating-point casts is improved. This may cause surprising
+  results for code that is relying on undefined behavior. Code sanitizers can
+  be used to detect affected patterns such as this:
+
+.. code-block:: c
+
+    int main() {
+      float x = 4294967296.0f;
+      x = (float)((int)x);
+      printf("junk in the ftrunc: %f\n", x);
+      return 0;
+    }
+
+.. code-block:: bash
+
+    clang -O1 ftrunc.c -fsanitize=undefined ; ./a.out 
+    ftrunc.c:5:15: runtime error: 4.29497e+09 is outside the range of representable values of type 'int'
+    junk in the ftrunc: 0.000000
+
+
 * Note..
 
 .. NOTE

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -495,7 +495,8 @@ namespace ISD {
     ZERO_EXTEND_VECTOR_INREG,
 
     /// FP_TO_[US]INT - Convert a floating point value to a signed or unsigned
-    /// integer.
+    /// integer. These have the same semantics as fptosi and fptoui in IR. If
+    /// the FP value cannot fit in the integer type, the results are undefined.
     FP_TO_SINT,
     FP_TO_UINT,
 

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -10890,6 +10890,15 @@ SDValue DAGCombiner::visitSINT_TO_FP(SDNode *N) {
     }
   }
 
+  // fptosi rounds towards zero, so converting from FP to integer and back is
+  // the same as an 'ftrunc': sitofp (fptosi X) --> ftrunc X
+  // We only do this if the target has legal ftrunc, otherwise we'd likely be
+  // replacing casts with a libcall.
+  if (N0.getOpcode() == ISD::FP_TO_SINT &&
+      N0.getOperand(0).getValueType() == VT &&
+      TLI.isOperationLegal(ISD::FTRUNC, VT))
+    return DAG.getNode(ISD::FTRUNC, SDLoc(N), VT, N0.getOperand(0));
+
   return SDValue();
 }
 
@@ -10929,6 +10938,15 @@ SDValue DAGCombiner::visitUINT_TO_FP(SDNode *N) {
     }
   }
 
+  // fptoui rounds towards zero, so converting from FP to integer and back is
+  // the same as an 'ftrunc': uitofp (fptoui X) --> ftrunc X
+  // We only do this if the target has legal ftrunc, otherwise we'd likely be
+  // replacing casts with a libcall.
+  if (N0.getOpcode() == ISD::FP_TO_UINT &&
+      N0.getOperand(0).getValueType() == VT &&
+      TLI.isOperationLegal(ISD::FTRUNC, VT))
+    return DAG.getNode(ISD::FTRUNC, SDLoc(N), VT, N0.getOperand(0));
+
   return SDValue();
 }
 

llvm/test/CodeGen/AArch64/ftrunc.ll

@@ -4,8 +4,7 @@
 define float @trunc_unsigned_f32(float %x) {
 ; CHECK-LABEL: trunc_unsigned_f32:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    fcvtzu w8, s0
-; CHECK-NEXT:    ucvtf s0, w8
+; CHECK-NEXT:    frintz s0, s0
 ; CHECK-NEXT:    ret
   %i = fptoui float %x to i32
   %r = uitofp i32 %i to float
@@ -15,8 +14,7 @@ define float @trunc_unsigned_f32(float %x) {
 define double @trunc_unsigned_f64(double %x) {
 ; CHECK-LABEL: trunc_unsigned_f64:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    fcvtzu x8, d0
-; CHECK-NEXT:    ucvtf d0, x8
+; CHECK-NEXT:    frintz d0, d0
 ; CHECK-NEXT:    ret
   %i = fptoui double %x to i64
   %r = uitofp i64 %i to double
@@ -26,8 +24,7 @@ define double @trunc_unsigned_f64(double %x) {
 define float @trunc_signed_f32(float %x) {
 ; CHECK-LABEL: trunc_signed_f32:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    fcvtzs w8, s0
-; CHECK-NEXT:    scvtf s0, w8
+; CHECK-NEXT:    frintz s0, s0
 ; CHECK-NEXT:    ret
   %i = fptosi float %x to i32
   %r = sitofp i32 %i to float
@@ -37,8 +34,7 @@ define float @trunc_signed_f32(float %x) {
 define double @trunc_signed_f64(double %x) {
 ; CHECK-LABEL: trunc_signed_f64:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    fcvtzs x8, d0
-; CHECK-NEXT:    scvtf d0, x8
+; CHECK-NEXT:    frintz d0, d0
 ; CHECK-NEXT:    ret
   %i = fptosi double %x to i64
   %r = sitofp i64 %i to double

llvm/test/CodeGen/ARM/ftrunc.ll

@@ -0,0 +1,42 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=armv7-eabi < %s | FileCheck %s
+
+define float @trunc_unsigned_f32(float %x) nounwind {
+; CHECK-LABEL: trunc_unsigned_f32:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    vmov s0, r0
+; CHECK-NEXT:    vcvt.u32.f32 s0, s0
+; CHECK-NEXT:    vcvt.f32.u32 s0, s0
+; CHECK-NEXT:    vmov r0, s0
+; CHECK-NEXT:    bx lr
+  %i = fptoui float %x to i32
+  %r = uitofp i32 %i to float
+  ret float %r
+}
+
+define double @trunc_unsigned_f64_i64(double %x) nounwind {
+; CHECK-LABEL: trunc_unsigned_f64_i64:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    .save {r11, lr}
+; CHECK-NEXT:    push {r11, lr}
+; CHECK-NEXT:    bl __aeabi_d2ulz
+; CHECK-NEXT:    bl __aeabi_ul2d
+; CHECK-NEXT:    pop {r11, pc}
+  %i = fptoui double %x to i64
+  %r = uitofp i64 %i to double
+  ret double %r
+}
+
+define double @trunc_unsigned_f64_i32(double %x) nounwind {
+; CHECK-LABEL: trunc_unsigned_f64_i32:
+; CHECK:       @ %bb.0:
+; CHECK-NEXT:    vmov d16, r0, r1
+; CHECK-NEXT:    vcvt.u32.f64 s0, d16
+; CHECK-NEXT:    vcvt.f64.u32 d16, s0
+; CHECK-NEXT:    vmov r0, r1, d16
+; CHECK-NEXT:    bx lr
+  %i = fptoui double %x to i32
+  %r = uitofp i32 %i to double
+  ret double %r
+}
+

llvm/test/CodeGen/PowerPC/fp-int128-fp-combine.ll

@@ -5,18 +5,7 @@
 define float @f_i128_f(float %v) {
 ; CHECK-LABEL: f_i128_f:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    mflr 0
-; CHECK-NEXT:    std 0, 16(1)
-; CHECK-NEXT:    stdu 1, -32(1)
-; CHECK-NEXT:    .cfi_def_cfa_offset 32
-; CHECK-NEXT:    .cfi_offset lr, 16
-; CHECK-NEXT:    bl __fixsfti
-; CHECK-NEXT:    nop
-; CHECK-NEXT:    bl __floattisf
-; CHECK-NEXT:    nop
-; CHECK-NEXT:    addi 1, 1, 32
-; CHECK-NEXT:    ld 0, 16(1)
-; CHECK-NEXT:    mtlr 0
+; CHECK-NEXT:    friz 1, 1
 ; CHECK-NEXT:    blr
 entry:
   %a = fptosi float %v to i128

llvm/test/CodeGen/PowerPC/fp-to-int-to-fp.ll

@@ -11,8 +11,7 @@ entry:
   ret float %conv1
 
 ; FPCVT-LABEL: @fool
-; FPCVT: fctidz [[REG1:[0-9]+]], 1
-; FPCVT: fcfids 1, [[REG1]]
+; FPCVT: friz 1, 1
 ; FPCVT: blr
 
 ; PPC64-LABEL: @fool
@@ -30,8 +29,7 @@ entry:
   ret double %conv1
 
 ; FPCVT-LABEL: @foodl
-; FPCVT: fctidz [[REG1:[0-9]+]], 1
-; FPCVT: fcfid 1, [[REG1]]
+; FPCVT: friz 1, 1
 ; FPCVT: blr
 
 ; PPC64-LABEL: @foodl
@@ -48,8 +46,7 @@ entry:
   ret float %conv1
 
 ; FPCVT-LABEL: @fooul
-; FPCVT: fctiduz [[REG1:[0-9]+]], 1
-; FPCVT: fcfidus 1, [[REG1]]
+; FPCVT: friz 1, 1
 ; FPCVT: blr
 }
 
@@ -61,8 +58,7 @@ entry:
   ret double %conv1
 
 ; FPCVT-LABEL: @fooudl
-; FPCVT: fctiduz [[REG1:[0-9]+]], 1
-; FPCVT: fcfidu 1, [[REG1]]
+; FPCVT: friz 1, 1
 ; FPCVT: blr
 }
 

llvm/test/CodeGen/PowerPC/ftrunc-vec.ll

@@ -4,8 +4,7 @@
 define <4 x float> @truncf32(<4 x float> %a) {
 ; CHECK-LABEL: truncf32:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    xvcvspsxws 0, 34
-; CHECK-NEXT:    xvcvsxwsp 34, 0
+; CHECK-NEXT:    xvrspiz 34, 34
 ; CHECK-NEXT:    blr
   %t0 = fptosi <4 x float> %a to <4 x i32>
   %t1 = sitofp <4 x i32> %t0 to <4 x float>
@@ -15,8 +14,7 @@ define <4 x float> @truncf32(<4 x float> %a) {
 define <2 x double> @truncf64(<2 x double> %a) {
 ; CHECK-LABEL: truncf64:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    xvcvdpsxds 34, 34
-; CHECK-NEXT:    xvcvsxddp 34, 34
+; CHECK-NEXT:    xvrdpiz 34, 34
 ; CHECK-NEXT:    blr
   %t0 = fptosi <2 x double> %a to <2 x i64>
   %t1 = sitofp <2 x i64> %t0 to <2 x double>
@@ -26,8 +24,7 @@ define <2 x double> @truncf64(<2 x double> %a) {
 define <4 x float> @truncf32u(<4 x float> %a) {
 ; CHECK-LABEL: truncf32u:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    xvcvspuxws 0, 34
-; CHECK-NEXT:    xvcvuxwsp 34, 0
+; CHECK-NEXT:    xvrspiz 34, 34
 ; CHECK-NEXT:    blr
   %t0 = fptoui <4 x float> %a to <4 x i32>
   %t1 = uitofp <4 x i32> %t0 to <4 x float>
@@ -37,8 +34,7 @@ define <4 x float> @truncf32u(<4 x float> %a) {
 define <2 x double> @truncf64u(<2 x double> %a) {
 ; CHECK-LABEL: truncf64u:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    xvcvdpuxds 34, 34
-; CHECK-NEXT:    xvcvuxddp 34, 34
+; CHECK-NEXT:    xvrdpiz 34, 34
 ; CHECK-NEXT:    blr
   %t0 = fptoui <2 x double> %a to <2 x i64>
   %t1 = uitofp <2 x i64> %t0 to <2 x double>

llvm/test/CodeGen/PowerPC/no-extra-fp-conv-ldst.ll

@@ -36,11 +36,7 @@ entry:
   ret float %conv1
 
 ; CHECK-LABEL: @foo
-; CHECK-DAG: fctiwz [[REG2:[0-9]+]], 1
-; CHECK-DAG: addi [[REG1:[0-9]+]], 1,
-; CHECK: stfiwx [[REG2]], 0, [[REG1]]
-; CHECK: lfiwax [[REG3:[0-9]+]], 0, [[REG1]]
-; CHECK: fcfids 1, [[REG3]]
+; CHECK: friz 1, 1
 ; CHECK: blr
 }
 
@@ -52,11 +48,7 @@ entry:
   ret double %conv1
 
 ; CHECK-LABEL: @food
-; CHECK-DAG: fctiwz [[REG2:[0-9]+]], 1
-; CHECK-DAG: addi [[REG1:[0-9]+]], 1,
-; CHECK: stfiwx [[REG2]], 0, [[REG1]]
-; CHECK: lfiwax [[REG3:[0-9]+]], 0, [[REG1]]
-; CHECK: fcfid 1, [[REG3]]
+; CHECK: friz 1, 1
 ; CHECK: blr
 }
 
@@ -68,11 +60,7 @@ entry:
   ret float %conv1
 
 ; CHECK-LABEL: @foou
-; CHECK-DAG: fctiwuz [[REG2:[0-9]+]], 1
-; CHECK-DAG: addi [[REG1:[0-9]+]], 1,
-; CHECK: stfiwx [[REG2]], 0, [[REG1]]
-; CHECK: lfiwzx [[REG3:[0-9]+]], 0, [[REG1]]
-; CHECK: fcfidus 1, [[REG3]]
+; CHECK: friz 1, 1
 ; CHECK: blr
 }
 
@@ -84,11 +72,7 @@ entry:
   ret double %conv1
 
 ; CHECK-LABEL: @fooud
-; CHECK-DAG: fctiwuz [[REG2:[0-9]+]], 1
-; CHECK-DAG: addi [[REG1:[0-9]+]], 1,
-; CHECK: stfiwx [[REG2]], 0, [[REG1]]
-; CHECK: lfiwzx [[REG3:[0-9]+]], 0, [[REG1]]
-; CHECK: fcfidu 1, [[REG3]]
+; CHECK: friz 1, 1
 ; CHECK: blr
 }
 

llvm/test/CodeGen/X86/2011-10-19-widen_vselect.ll

@@ -71,8 +71,7 @@ define void @full_test() {
 ; X32-NEXT:    subl $60, %esp
 ; X32-NEXT:    .cfi_def_cfa_offset 64
 ; X32-NEXT:    movsd {{.*#+}} xmm2 = mem[0],zero
-; X32-NEXT:    cvttps2dq %xmm2, %xmm0
-; X32-NEXT:    cvtdq2ps %xmm0, %xmm1
+; X32-NEXT:    roundps $11, %xmm2, %xmm1
 ; X32-NEXT:    xorps %xmm0, %xmm0
 ; X32-NEXT:    cmpltps %xmm2, %xmm0
 ; X32-NEXT:    movaps {{.*#+}} xmm3 = <1,1,u,u>
@@ -93,8 +92,7 @@ define void @full_test() {
 ; X64-LABEL: full_test:
 ; X64:       # %bb.0: # %entry
 ; X64-NEXT:    movsd {{.*#+}} xmm2 = mem[0],zero
-; X64-NEXT:    cvttps2dq %xmm2, %xmm0
-; X64-NEXT:    cvtdq2ps %xmm0, %xmm1
+; X64-NEXT:    roundps $11, %xmm2, %xmm1
 ; X64-NEXT:    xorps %xmm0, %xmm0
 ; X64-NEXT:    cmpltps %xmm2, %xmm0
 ; X64-NEXT:    movaps {{.*#+}} xmm3 = <1,1,u,u>